At launch? Mario 64. It was totally different to anything else.
I remember going to the local Blockbuster at every opportunity prior the the N64's launch; they had a station set up with the game. God knows how many times I played through the first couple of stars.
OoT came out a couple of years later, so it didn't have the same 'wow' factor as Mario 64 had. I'd been playing around in big 3D worlds for a while by then, and had already played through Mystical Ninja, which did a lot of the things OoT did before OoT did them.
Nowadays I'd take OoT over Mario 64 though. Really magical game!







