Well, first you have to define "the best". From statistical point of view, you have to options:
a) The biggest share of winnings is won by Americans
b) The biggest share of American players are winning players
A is true but at the same time B is most likely wrong. I'm speaking merely from my FLSH experiences where tables fill up with fish when Americans wake up, but I think I could generalize that to all game types up to somewhat medium or high limits.
The reason was already pointed out - Poker is super popular in USA so everyone with internet connection is playing it even if they don't know excactly what they're doing but Tom Dwan seemed really cool check/raising with air on river.
On the other hand people from smaller country tend to be better players in general because Poker is not yet so widely spread and everyone getting in touch with Holdem is very often introduced to it by some friend or acquintant who already plays it succesfully and therefore a newby is going to learn first instead of just dumping all the chips on the first table he runs to. Of course that is not the case with all the players, but certainly the percentage of that kind of players is bigger than in much bigger/richer countries.
I can tell from my personal experience that there is a big amount of succesful Slovenian FL players and whenever I ran into random Slovenian on the tables they were playing solid poker at least. The only exception is Germany which produces a vast amount of solid low-medium limit players, but I believe Pokerstrategy could have something to do with that.