Floyd Mayweather hasn't had a truly close fight in over a decade and Andre Ward has never had one. So it might be a bit of an exaggeration to argue that there is a "nightmare" fight out there for either man.
But there are certainly opponents available who would make for compelling matchups with both undefeated champions.
Sugar Ray Robinson was the greatest pound-for-pound boxer of all time, but Jake LaMotta was consistently able to push him to his limit. Muhammad Ali may truly have been "The Greatest," as he claimed, but Ken Norton and Joe Frazier were both able to beat him once and give him close to all he could handle in rematches.
No fighter is invincible. Not even the all-time greats.
Aside from Floyd Mayweather and Andre Ward, no fighter has looked more untouchable of late than Guillermo Rigondeaux. In April the two-time Olympic gold medalist handed pound-for-pound star Nonito Donaire a boxing lesson, in just his 12th professional fight.
But Leo Santa Cruz would present a different kind of problem for Rigo than Donaire did. Donaire is an explosive puncher who frequently hangs back, waiting for the opening to deliver the big shot. Against Rigondeaux he had zero chance of getting that opening.
The one round when Donaire truly went on the offensive, in the 10th, he managed to catch up to Rigondeaux and knock him down.
Santa Cruz is a relentless swarmer with a solid chin and excellent offensive footwork. Rigondeaux is a master at controlling distance, but Santa Cruz's length and reach advantages could complicate that for Rigo.
I'd definitely be inclined to pick Rigondeaux if these two ever fought. But it would be an extremely compelling matchup with a lot of potential for fireworks.
Unfortunately, Rigondeaux is promoted by Top Rank and Santa Cruz by Golden Boy, so this is a fight we are very unlikely to see.
Sergio Martinez is one of the most exciting boxing stars of the past decade. But the next time he gets in the ring he will be 39 and coming off from multiple surgeries.
A monster puncher with excellent boxing skills like Gennady Golovkin would be a very dangerous opponent for him. Golovkin cuts off a ring expertly. He throws one big punch and is always in position to unload the next one.
I'm more inclined to think Martinez will fight Miguel Cotto in his return bout. At this point in his career I can't blame him for taking a much safer fight that will likely earn him more money.
But if he doesn't fight Golovkin some time next year, it will be the equivalent of ceding GGG the middleweight division. The former Olympian from Kazakhstan is one of the hottest fighters in the sport and the temperature around him goes up each time he fights.
This was probably the easiest selection of the bunch. There's a reason that Manny Pacquiao and Juan Manuel Marquez have created the greatest boxing rivalry of their generation. They are both extremely good at doing things that give the other man fits.
In their four fights, Pacquiao's foot speed has consistently allowed the southpaw to get advantageous angles on his greatest rival. His explosiveness has frequently allowed him to reach Marquez with punches before the defensively adept Marquez can react.
But just as often Marquez's precise and efficient footwork has been able to negate Pacquiao's speed. At times in their fights Pacquiao has taken three steps while Marquez takes just one, only to find that Marquez's one step has put him in a better position.
Marquez also has an uncanny ability to time Pacquiao's punches and disrupt them with preemptive counters.
In their four-fight series there has been one draw and split-decision and majority-decision victories for Pacquiao. Marquez won their fourth fight decisively, by a KO at the end of Round 6.
But at the time he was put to sleep Pacquiao was a mere second away from taking a four-round to two lead on all three cards.
If these two end up meeting for a fifth time in 2014 expect another tense, evenly contested war.
In the past two years Kubrat Pulev has established himself as the obvious No. 1 contender to Wladimir Klitschko's heavyweight crown. In 2012 he knocked out Alexander Ustinov and Alexander Dimitrenko, two gigantic, highly rated contenders with a combined record of 59-1.
In 2013 he beat longtime heavyweight contender Tony Thompson by unanimous decision and knocked out journeyman Joey Abell.
Pulev has the size to prevent Klitschko from using the same clinch-and-lean strategy that he employed against Alexander Povetkin in October. He also represented Bulgaria in the 2008 Olympics and has the technical boxing ability to reach the champion's notoriously vulnerable chin.
Boxing fans have been waiting years for a heavyweight to emerge who represented a legitimate challenge to Klitschko. Pulev looks like he has the tools to be that guy. If he doesn't get his shot in 2014 it will be terrible for the division and the entire sport.
Timothy Bradley proved he is an elite boxing talent when he beat Juan Manuel Marquez last October. But last March he was nearly stopped in each of the first two rounds by Ruslan Provodnikov's power punching. Only Bradley's tremendous endurance and heart allowed him to escape with the win.
Bradley made some tactical mistakes in that fight that he's unlikely to make again. But his lack of fight-ending power also allowed Provodnikov to hang around long enough to knock him down and nearly end the fight again in Round 12.
Keith Thurman is far less experienced than Bradley. Still, "One Time" would enjoy a significant power advantage over the WBO champion, and unlike Marquez, he is a natural welterweight.
After climbing the ranks by steamrolling lesser talents, Thurman has shown patience and the ability to make adjustments while knocking out his more recent opponents. Thurman looks like a very well-rounded fighter.
Unfortunately, this is yet another great fight we are unlikely to see, due to the Top Rank-Golden Boy Cold War.
I think the more common choice for Andre Ward would be Gennady Golovkin. That is certainly a fight I would love to see and since Ward and Golovkin are both HBO fighters, I wouldn't be shocked to see it get made.
But eventually I think Ward is going to end up moving up to light heavyweight to fight Sergey Kovalev, too. And that might potentially be an even tougher fight for him.
Like Golovkin, Kovalev is a ferocious puncher who is a master at tactically cutting off the ring and trapping an opponent. But unlike GGG, Kovalev is bigger than Ward.
A smart technical fighter like Ward capitalizes on opponents' mistakes, but Kovalev isn't the kind of fighter who makes a lot of mistakes. He certainly gets hit on occasion, but almost always when he's in position to deliver a bigger punch in return.
Ward is a special talent, but he's still human. A monster puncher who can get in position to hit him is going to have a shot to win.
Like any boxing writer, I would love to see Floyd Mayweather vs. Manny Pacquiao finally get made. For almost anybody associated with the sport, the fight would be good for business.
But I also believe Mayweather would end up winning against Pacman with relative ease. I think a much tougher matchup for the pound-for-pound king would be the interim WBA light middleweight champion, Erislandy Lara.
Lara, like Guillermo Rigondeaux, is a product of the Cuban amateur program. He is a southpaw with elite boxing skills and a high ring I.Q.
For years the so-called game plan for beating Mayweather has been to crowd him and rough him up. But Mayweather has consistently made a mockery of this strategy. With his sublime shell defense he is just as tough to hit when he's right in front of you as he is moving laterally on the outside.
Lara is a patient counter puncher. Mayweather's bread-and-butter is walking an opponent into his stinging lead right. That would be no easy task against Lara.
Mayweather deserves his pound-for-pound status. He has been a wizard at making tactical adjustments and he'd certainly be the favorite against Lara.
But I think it's his toughest potential fight below 160 pounds.