3D World, easily, for me. I've replayed many levels in that game over and over again. It feels like what the jump to 3D always should have been for Mario.
I REALLY like Sunshine, too, though. Such an underrated title.
I DO NOT like Super Mario 64. For 3 big reasons: It's empty. There's a lot of levels that just feel too open without a lot going on. That isn't exactly a game-killer for me, though. The next two-reasons are: Having to replay levels multiple times...I hate that. I know, they did it on Sunshine, too, but in that game enough changed on the level each time and there was enough stuff going on in the levels to keep me occupied. But the biggest offender though, which isn't exactly the game's fault, is that damn controller. Still hate it.