The series is of extraordinary quality, so the ranking is cherry picking heavy. Anyway, I'd rank them as such:
Super Mario 64
Super Mario Galaxy
Super Mario Galaxy 2
Super Mario 3D Land
Super Mario Sunshine
Super Mario 64 is my #3 best game of all time, so it had to win this. Maybe it's because I played this in it's launch week back in '96 for the first time, maybe not. Fact is that this game is the definition of fun. Gameplay, level-design, graphics (for it's time), sound; all were excellent. In the top 50 best games thread last December, I included all of them, except Sunshine, which I find the least of the series. I don't mind colorful, but Sunshine was just a little too 'childish'. Still a great game though!