Galaxy 2, by a hair.

I've developed a bit of ADD with games as I've gotten older/amassed too many of them, so I haven't played all the way through Galaxy 2 yet. I have played though the story in the first one, but have not completed all the little stuff. (Incidentally, I still haven't even managed to see Sunshine all the way through to Bowser. Even though it makes perfect sense for a plumber to man a water jet system, the resulting severe learning curve makes Sunshine a chore. That said, it's more laziness than anything else)

The first Galaxy was a real return to form for Nintendo, and a real benchmark game for the series. It isn't perfect, but I consider it a truly triumphant title. There are some pretty electrifying moments throughout. It's also absolutely stunning both visually and aurally.

However, despite lacking the impact advantage the first had, Galaxy 2 manages to best the original by somehow recapturing some of the magic Super Mario Bros. 3 had as a sequel. I think it really boils down to the abundance of truly well designed & fun Mario suits & gimmicks. The choice to dispense with the hub world concept of 64, Sunshine & the Galaxy in favor of a map system is another reason. It is also more lighthearted, and less bogged down in exposition than the original.

For my taste, it's a better video game.