What is the best mario game in your opinion
Next time, please expand on your own post with your opening comments. Posting a one-liner topic opener just comes off as... lazy =/
Anyways, it really depends on which gaming genre we're talking about. Mario is such a varied franchise now that I really can't pick one favourite. I can pick a few, though.
As far as the platformers go, Yoshi's Island is probably my favourite. I love SMB 3 and Super Mario World, but Yoshi's Island just felt like a more refined game all-around. The levels are nicely designed (open and offering lots of exploring), it has a nice percentage completion system for each level (giving you a reason to replay them for that 100% score), hidden stuff to seek out, little bonus games... and lots of Yoshi. I love Yoshi. But even without the Yoshi element, the game was excellent.
Then we have the racers... I do love my Mario Karts. Mario Kart 64 is probably my original favourite, but at the moment Mario Kart Wii is the one I play the most. Using the wheel is just really fun, and I like the new trick system.
Lastly, the RPGs. I adore Mario & Luigi: Partners in Time, Super Mario RPG, AND Paper Mario: The Thousand-Year Door. They're all played slightly differently, but each share a similar Mario flair and humor. They're just awesome games even outside the Mario franchise. The Thousand-Year Door, in particular, was gorgeous to look at, too.
I've never been a huge fan of the 3D Mario games, but I'd stick Galaxy at the top of my list for the astronomy elements and the interesting controls. I'm also not a big fan of NSMB - found it far too easy and nothing standout-ish from one world to the next. I'm hoping the Wii version leaves a bigger impact.