Mario 64 sucks

It's the game that killed any interest I ever had in the Mario franchise. And by extension, to some extent, Nintendo as a whole.

The shift to 3D left me completely cold - both Mario 64 and Mario Kart 64 were terrible to me (both looking and playing like shit) compared to the SNES games in those franchises. Added to the fact that I hated Ocarina of Time for similar reasons, and the fact that Final Fantasy jumped to the PS1 as a franchise, it's why I basically shifted from Nintendo to Sony and never looked back.

Since then, Nintendo's kept going in directions that are either completely disinteresting to me or are outright annoying, while my fickle taste tends to wobble back and forth between Playstation and Xbox these days. It didn't help that almost every game I ever wanted to play or that interested me either wasn't on the current-gen Nintendo console at all, or if it was it was also available on the others as well. So I've never felt the need to own one ever again.

There are very few games I can point to and say "THIS is why I don't own Nintendo consoles anymore", but Mario 64 is definitely one of them (Ocarina's probably another, and Shadowgate 64 probably falls in that list as well). It forms a pretty clear dividing line in my brain for the moment when I stopped caring about Nintendo.