1) Nick Franklin, SS/2B
Franklin stated his case as the best offensive shortstop in baseball last year, hitting 23 homers while maintaining a .283 average. He also stole 25 bases and played solid enough defense to make the M's consider leaving him at shortstop. Franklin impressed everyone by reaching Double-A before his 20th birthday, but he'll actually start off this season in High-A ball.
2) Taijuan Walker, RHP
With the inevitable graduations of Michael Pineda and Dustin Ackley, the Mariners are pretty bereft of elite talent. Aside from Franklin, the team's best bet is Walker, who was the team's first selection in the 2010 draft.
He was one of the most raw pitchers taken in the first two rounds, and the M's will have their work cut out for them, but Walker has immense talent, including a mid-to-high-90s fastball and a plus-curveball, so things are looking promising.
3) Guillermo Pimentel, OF
Pimentel was the team's most heralded signing of the 2009 international class, and while he struggled in his debut, he flashed some of the best power to come along in Seattle's system in many, many years. He hit six homers in 51 games in the AZL, and has 30-HR potential going forward. His biggest drawback is his inability to recognize breaking pitches. He struck out a whopping 58 times in 184 at-bats in 2010.
4) Marcus Littlewood, SS
While Franklin might eventually have to move to second base, Littlewood has all the tools and polish to stick as the team's long-term shortstop. In addition to being solid on defense, Littlewood also has an advanced approach at the plate, has decent speed and is one of the hardest workers you'll find in the Mariners system. He is off to a good start in 2011, hitting .455 through three games at Low-A Clinton.
5) Alex Liddi, 3B/1B
The Mariners have been the biggest international player in baseball over the past decade and a half. They've produced big leaguers from all over the Caribbean, as well as the Far East. With Liddi, they're branching out into the Mediterranean.
Hailing from Italy, Liddi is one of the more polished players in the system. He hit 23 homers in 2009 and another 15 last year, adding 92 RBI. He's a polished hitter with a free-swinging attitude that results in big strikeout numbers, but he's also good for 20–25 homers per year as a big leaguer. Liddi's in Triple-A for the first time in 2011.