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Here are the top 20 deals of the 2015 January transfer window. 

The 20 permanent transfers will be ranked subjectively via four main determinants: 

• Current playing ability: Is the footballer of a world-class, top-flight standard, a rising star or a squad player? 
• Potential playing ability: How high is the footballer's upside? 
• Transfer economic efficiency: Is the deal value for money?
• Projected transfer resell value: What is the likelihood of the buying club selling their new player down the line for a profit?

The main list only ranks permanent transfers, meaning loans were excluded.

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10 Honourable Mentions and Loans

1. Spanish right/left attacking midfielder Suso, 21, Liverpool → AC Milan: €1.3/£1 million        
2. German central/right attacking midfielder Hany Mukhtar, 19, Hertha Berlin → Benfica: €500,000/£377,328
3. Norwegian left-winger Mats Moller Daehli, 19, Cardiff City → Freiburg: €1.5/£1.1 million
4. French central midfielder Clement Chantome, 27, Paris Saint-Germain → Bordeaux: €750,000/£565,992
5. Chilean central midfielder Marcelo Diaz, 28, Basel → Hamburg: €2/£1.5 million
6. Argentine left-forward Lucas Ocampos, 20, Monaco → Marseille: Loan
7. Serbian centre-back Matija Nastasic, 21, Manchester City → Schalke: Loan
8. Italian centre-forward Mattia Destro, 23, Roma → AC Milan: Loan
9. Croatian central/central attacking midfielder Marcelo Brozovic, 22, Dinamo Zagreb → Inter Milan: Loan
10. Serbian central attacking midfielder Filip Djuricic, 23, Benfica → Southampton: Loan

There is no ethical issue when you compile a list of transfer rumours and rudimentarily rank it.

But it is a different ballgame when it comes to ranking completed transfers on a competency scale. 

I have not watched the following 10 players enough to form a solid opinion on their proficiency; hence, their deals were omitted from the main list. 

Ranking players you don't really know about is like the Left Shark (I am assuming you watched the Super Bowl) trying to keep up with Katy Perry's dance choreography.

10 Omissions

1. Brazilian defensive midfielder Lucas Silva, 21, Cruzeiro → Real Madrid: €14/£10.6 million
2. Argentine left/central attacking midfielder Joaquin Correa, 20, Estudiantes → Sampdoria: €6.5/£4.9 million
3. English central midfielder Dele Alli, 18, Milton Keynes Dons → Tottenham Hotspur: €5/£3.8 million
4. French right-winger Sofiane Boufal, 21, Angers → Lille: €4/£3 million
5. Polish defensive midfielder/centre-back Krystian Bielik, 17, Legia Warsaw → Arsenal: €3.2/£2.4 million
6. Norwegian central attacking midfielder Martin Odegaard, 16, Stromsgodset → Real Madrid: €3/£2.3 million
7. English central midfielder Matt Grimes, 19, Exeter City → Swansea City: €2.32/£1.75 million 
8. Belgian left/right-forward Theo Bongonda, 19, Zulte-Waregem → Celta Vigo: €1.3 million/£981,053
9. Swedish centre-forward Carlos Strandberg, 18, Hacken → CSKA Moscow: €500,000/£377,328
10. French centre-forward Maxwell Cornet, 18, Metz → Lyon: €400,000/£301,680

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Espanyol → Villarreal: €5.7/£4.3 million

Ivorian centre-back Eric Bailly, 20, is physically blessed at 6'2" and 170 pounds.

In his limited appearances for Espanyol, Bailly threw himself into tackles and was constantly trying to cut off passes.

His 92.3 tackle-success percentage is a big plus for Villarreal.

However, he is raw, and it is a reach considering he has only played 354 minutes of La Liga action this season.

Considering Villarreal bet correctly on Gabriel Paulista, you surmise Bailly is another potential cash cow.

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Malmo → RB Leipzig: €3.7/£2.8 million

Swedish left/central midfielder Emil Forsberg, 23, was the pick of the bunch when it came to Malmo's players in the UEFA Champions League.

Forsberg was dedicated to tracking back and provided some spark going forward.

He is a smart transfer for RB Leipzig, a team with aspiration for promotion to the German Bundesliga.

In the past, Germany has been a destination for Malmo players:

• 2014: Jiloan Hamad (Hoffenheim).
• 2014: Miiko Albornoz (Hannover 96).
• 1995: Jorgen Pettersson (Borussia Monchengladbach).
• 1991: Martin Dahlin (Borussia Monchengladbach).
• 1966: Bosse Larsson (Stuttgart).
• 1966: Lars Granstrom (Karlsruher).

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Benfica → Valencia: €25/£18.9 million

Argentine central midfielder Enzo Perez, 28, is a footballer who can enhance any team with his ball-winning prowess and accurate passing.

However, Valencia do realise he is 28 years old, right?

He has limited upside, and unless he is an iron man when it comes to his fitness, it is a big risk to pay such a lofty amount. 

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Rijeka → Leicester City: €12.6/£9.5 million

Croatian centre-forward Andrej Kramaric, 23, is an ambitious signing from Leicester City.

He has scored 21 goals in 18 league games and has five goals in the UEFA Europa League, so his statistics are world-class.

However, Kramaric's start hasn't been the best for Leicester City.

He has played as if he is Bosko Balaban rather than the free-scoring No. 9 at Rijeka.

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Fiorentina → Chelsea: €30.9/£23.3 million plus Mohamed Salah on loan, according to BBC Sport

Colombian utility player Juan Cuadrado, 26, is a bizarre signing given his circumstances.

Yes, he is a freak athlete, he can take on opposing defenders and his playing style fits the English Premier League.

But he has transitioned into a deep-lying forward, yet Chelsea are presumably expecting him to be a right attacking midfielder. 

He has a reputation as a complete footballer, though that has not been the case this season.

He averages more fouls per game (1.4) than tackles made (1.2), so he needs to drastically improve his defending, otherwise he will be a liability.

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Chelsea → Wolfsburg: €31.8/£24 million, according to the Daily Mail's Rajvir Rai

German right attacking midfielder Andre Schurrle, 24, is a wasteful shooter and was not utilised correctly by Chelsea. 

Germany integrate him into their system, whereas Chelsea viewed him as just another toy, which is probably why he never truly believed in himself.

This is a FIFA World Cup winner who underperformed at Chelsea despite his speed and direct dribbling. 

Sure, he is a fantastic player when he is in form.

But for Wolfsburg to pay €31.8/£24 million for Schurrle based on his reputation in Germany is outlandish and flagrantly inefficient.

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Red Bull Salzburg → Borussia Dortmund: €12/£9.1 million

Slovenian right attacking midfielder Kevin Kampl, 24, has all the moves, skills and confidence to be on the books of Borussia Dortmund.

But there are question marks over his productivity. 

Beating up on Astra Giurgiu in the UEFA Europa League should not be a plus; it should be expected.

He was OK starting on the right in Borussia Dortmund's 0-0 draw against Bayer Leverkusen but tended to drift centrally.

He had moments of genius but failed to score or create a goal. 

Kicker rated him as the best of Borussia Dortmund's attackers (on a scale of one-to-six; one being world-class, six being awful).

Kampl needs to start producing, because Borussia Dortmund signed him basically as an insurance package to avoid relegation.

Borussia Dortmund are currently bottom of the German Bundesliga.

Benfica → Monaco: €15.75/£11.89 million

Portuguese central attacking midfielder Bernardo Silva, 20, is elegant on the ball and looks like a world-class player.

But his production is not good enough: two goals and one assist in 17 Ligue 1 games.

That may have swayed Benfica to let Silva go, even though his technique would be in the top 1 percent.

Monaco probably believe Silva can be to them what Deco, who was also let go by Benfica, was to Porto.

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Swansea City → Manchester City: €37.1/£28 million

Ivorian centre-forward Wilfried Bony, 26, is a brilliant No. 9.

But to pay €37.1/£28 million for someone who has scored nine goals in 20 Premier League games this season is absurd.

He is essentially a luxury injury-replacement player should Sergio Aguero's body break down again.

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CSKA Moscow → Roma: €14.3/£10.8 million

If Ivorian centre-forward Seydou Doumbia, 27, was eight years younger, this deal would be No. 1 on this list.

Drake's "Started from the Bottom" would sum up Doumbia's rise, because he worked his way up from obscurity, having played in Japan.

It does make you wonder how many other gems are hidden away in the Japanese J. League.

Doumbia is a one-man counter-attack and will be a nightmare for opposing defences in the Italian Serie A, especially if combined with Victor Ibarbo. 

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Sampdoria → Napoli: €13/£9.8 million

Italian right forward Manolo Gabbiadini, 23, is a gritty shooter with loads of confidence.

He loves shooting from range, and if given space he can punish opposing teams.

He scored seven times from 13 Serie A games for Sampdoria, so Napoli are signing a productive footballer.

His winning goal in Napoli's 2-1 win over Chievo Verona could be a sign of things to come.

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Villarreal → Arsenal: €14.9/£11.2 million plus Joel Campbell on loan

Brazilian centre-/left-back Gabriel Paulista, 24, is an intriguing signing, and "paradox" is one word to describe him.

He averages 3.5 interceptions per game, so you would think his positioning is outstanding, yet he routinely gets caught out not marking opposing players.

Given Gabriel's athleticism and his ability to learn quickly (he has made big strides since coming to Europe), it is understandable why Arsenal have taken a calculated gamble on him.

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Roma → Bayer Leverkusen: €5/£3.8 million

Croatian centre-/right-/left-back Tin Jedvaj, 19, has been inconsistent and is attempting to figure out what type of defender he is. 

There is a Sergio Ramos-esque vibe to Jedvaj's often frenetic defending, which can lead to impulsive actions and put Bayer Leverkusen at risk of conceding a goal.

That said, Jedvaj is an immense talent, and Bayer Leverkusen are willing to nurture him despite his errors.

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Chelsea → Southampton: €13.3/£10 million, according to The Sun

What happened to English players' transfer stocks being inflated? 

English left-back Ryan Bertrand, 25, is having a career season at Southampton.

He has been a mainstay at left-back and has contributed to Southampton's having the best defence in the English Premier League (17 goals conceded from 23 games).

This is why €13.3/£10 million is a fair fee, especially when Luke Shaw is valued at €39.8/£30 million by Manchester United.

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Bayern Munich → Inter Milan: €19.9/£15 million

Swiss right attacking midfielder Xherdan Shaqiri, 23, must have complained his way out of Bayern Munich.

Inter Milan have won out on this deal.

Given how inflated the transfer market is, €19.9/£15 million is a discounted price for Shaqiri, a player with world-class potential.  

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Newcastle United → Roma: €7.3/£5.5 million

French centre-back Mapou Yanga-Mbiwa, 25, was a pawn in Newcastle United owner Mike Ashley's dogmatic and erratic reign. 

Mapou might be thinking to himself, "Thank goodness I'm at Roma, otherwise I'd probably be at Rangers now."

It is astounding how Ashley and the club's management have overlooked Mapou's physical ability and leadership qualities. 

Newcastle United's loss is Roma's gain.

Hoffenheim → Werder Bremen: €2.5/£1.9 million

Danish centre-back Jannik Vestergaard, 22, is a defender with a towering presence at 6'6" and 216 pounds.

Sure, Vestergaard does not have a higher upside than Hoffenheim's Niklas Sule, who is a big prospect but is rehabilitating from an anterior cruciate ligament tear.

But it is dicey for Hoffenheim to give up on Vestergaard, whose problem isn't quality but consistency.

In Werder Bremen's 2-0 win over Hertha Berlin, Vestergaard made three tackles, intercepted four passes and cleared the ball four times. 

If Vestergaard can keep up his performances, Werder Bremen may have found an elite centre-back.

Manchester City → Groningen: €550,000/£414,810.

Slovakian central midfielder/left-winger Albert Rusnak, 20, can morph into different types of players. 

He can beat opposing defenders as a dribbler, he is comfortable on the ball to be a passing outlet, and he shows zeal in trying to win back the ball. 

From Cambuur to Groningen, Rusnak is intent on maximising his ability.

With his attitude and all-round playing style, his upside is extremely high.

Don't be surprised if Manchester City bring back Rusnak when he is a more polished footballer, just like Chelsea did with Nemanja Matic.

Djurgardens → Groningen: €1.3 million/£980,460.

Swedish central midfielder Simon Tibbling, 20, plays as if he has the gravitas of a Dusan Tadic or a Luis Suarez.

Three games into Tibbling's Dutch Eredivisie career, he is already a ball-magnet, completing 90.9 percent of his 77 passes per game.

It goes to show how confident the Groningen players are in Tibbling's elite passing ability.

Tibbling is a special player and Groningen have unearthed a diamond in the rough.

Heerenveen → Ajax: €6.5/£4.9 million

Dutch central midfielder Daley Sinkgraven, 19, is a prospect with a comprehensive playing style.

He is still a teenager yet is already a more complete footballer than many seasoned professionals in the Dutch Eredivisie.

Ajax management are probably hoping Sinkgraven's success at the club will be more akin to that of Klaas-Jan Huntelaar than Miralem Sulejmani.

+allanjiang.

When not specified, statistics via WhoScored.com.