Both Nakamura and Fischer are uncompromising players who take a lot of chances to win, and so they end up losing quite a few games. Of the American players, Boris Gulko and Samuel Reshevsky were probably the hardest to beat.
namenohad - Karpov came in at 18.0%.
I suspected he couldn't have been far off of that list...
I can't believe nobody mentioned Capablanca yet.