I like them all...
Super Mario 64 was good, yet it bores me when I think of its obscure levels and hub. (You feel trapped inside a castle)
Super Mario Sunshine was awesome. A bit boring, but still awesome.
Super Mario Galaxy is perfect. I beat it already (Yes, my file is sparkling, therefore I beat it completely), and it hints at a sequel. Nice too.
I voted Galaxy. It's fun, pure Mario and...epic!