Id say take a look at the seasonal TT boards and notice the handful of guys who seem to be able to always every TT there is get in top ten and sometimes win well more than sometimes.
Yeah I know there could potentially be some slacker GT5 player who is so dang good he got bored of the game and rarely plays. But you can pick one of those guys as overall best I would say.
I know L_Shooter from Japan is awesome that dude has always placed top ten in almost every seasonal TT since I got GT5 in february and occassionally he gets the win. He also placed top ten in the drift trials.
He uses wheel for the TTs and a DS3 for the drifting. All around hes one of the best.
Ive placed top ten but that dudes in a whole nother league and a half.
There are many others as good but are they overall better? Like I said I can only remember one TT he didnt place top ten in and it was because japan couldnt enter it, there PSN was still down. hahaha