First off, all three players are on teams with very good records. Boston, Cleveland, and Orlando are all winning their divisions by huge margins and all three have taken turns as the team with the best record in the NBA. With that out of the way, we can concentrate on stats. If you only consider scoring, then you'd rank them Allen (18.2), Williams (17.1), and Nelson (16.7). Nelson leads the group in FG% 50.3%, then Allen with 49.5%, and finally Williams at 46.4%. The FG% of Williams is pretty good for a guard, but Nelson and Allen's numbers are huge. Usually you only see those kind of numbers from guards who do not show 3pters, a la Tony Parker. However Nelson and Allen are both averaging more than two 3pters made per game, and both shooting over 40% on 3pters. Based on their ability to shoot and score, it's very hard to argue Mo Williams over either Allen or Nelson.
Basketball is more than just shooting, though. Williams averages 4.2 assists per game. That is better than Allens' 2.8 assists per game, but less than Nelson's 5.4 assists per game. However Williams is the worst of the group in turnovers, at 2.3 per game. Nelson only has 2.0 TOs/game and Allen 1.8 TOs/game. So based on ball handlings, maybe you could argue Williams over Allen.
What about defense? The only stat we can go on here is steals. Again Nelson is the best with 1.2 steals/game, then Allen at 1.0 and Williams at 0.8. So it is crazy to argue Williams over Nelson. Nelson is a better scorer, ball handler, and defender. To argue Williams over Allen, you can only consider ballhandling. Allen is better in scoring and defense (rebounding too, but these are guards.)
The final argument I've heard for William has nothing to do with Williams or the people chosen over him. It goes "how can Cleveland have only one All-Star when they have such a good record?" Perhaps the better question is "how can Cleveland have such a good record when they only have one All-Star?"