Mario Galaxy 1.
Don't get me wrong, 2 has better highs. But 2 feels like a game with "better level design," not a game with more charming, timeless content. 2 also feels incredibly factory-made and level-pack-ish in comparison (Chompworks, boss gauntlet, etc.). It was clearly designed as a follow-on to the original.
Galaxy 1, on the other hand, blew the roof off the genre, and seems like it was designed from the start to be a redeeming light for the 3D Mario series. (It was successful, by the way.)
Galaxy 1 also completely justified its space adventure theme; Galaxy 2 didn't really feel like it tried very hard - Starship Mario was a poor follow-up to the Comet Observatory, and the world map structure, while convenient, felt really washed-up compared to the creative "observatory dome" thing going on in the first game.