Super Mario 64.
I understand that it was an achievement in 3D gaming, with the open level design. But the game was just bland, and boring to me. And I HATE that you had to replay the same levels over-and-over again, which made it all much more boring.
The other platformers of the time were much more fun, IMO. The Crash Bandicoots, the Spyros, Pac-Man World, Gex (the first game at least), and Rayman.