Man this is going to be tough. I'd consider a Sonic the Hedgehog game, and by Sonic the Hedgehog I mean one of the Genesis classics, not the newer and cruddier games like Unleashed. Man, that game is an absolute abomination! But I'm not going to talk about how bad recent Sonic games have been.
Anyways, the main candidates for my favorite platformer are Super Mario Galaxy, Super Mario Galaxy 2, and Super Mario World.
Now, out of the three, Super Mario Galaxy is in my opinion my least favorite. Sure, the whole gravity aspect and all the cool space travel is awesome, but I feel that SMG2 pretty much made every aspect of the game better, including the comets. However, it also has a great story for a Mario game. It's nowhere near as great as Final Fantasy V as an example, but it's incredible for a Mario game.
SMW is probably my second favorite of the three because it has very diverse levels and it's actually pretty tough (curse you Bowser's Castle, CURSE YOU!). It also had great music (Koopa Kids, a lot of the level themes, Bowser Battle) and even today I say it's got some pretty good graphics in comparison to games. The Map was awesome too, it's actually the only Mario game I know of where the worlds are all just one big super image. They never did that with any of the other games.
SMG2 for the reason I listed above, but it also was rediculously hard if you ask me. Way harder than SM64 if you ask me and there were quite a few levels where I was dying nonstop and even some of the easier levels made me die. Hooray for 1-Up Mushrooms! The music, as with the other two games, was incredible and it produced a
very favorite song in all of gaming (definately not an exaggeration) for me.
So yeah, SMG2 is my favorite with SMG1 and SMW trailing close behind.