Most fun I've had with Mario since Super Mario 64
Super Mario Galaxy is an absolute gem of a platformer. Sure it might seen a little out of the box with a Mario game that is set in space, however the gameplay you experience is almost unprecedented. I haven't had this much fun with a video game in a LONG time and that's saying quite a bit.
There is a great deal of variety in gameplay mechanics as well as just the right amount of challenge and collection pieces to not be too easy or too hard. The graphics are great with amazing colors that really catch your eye, presenting a wonderful experience and environment to play along in.
If you love Mario or platformers and you haven't played this game. Please, do yourself a favor and just go buy it.. NOW!