Battlefield and Smashville for Mario, but I'm sure that's common knowledge for anybody that's ever seen a Mario play.
Battlefield's platforms are arranged in such a way that it really helps with Mario's combos and aerials, and Smashville's platforms is nice to get some throws going. Up Throw>Up Air>Jump>Up air and Up B is a really deadly combo in that platform.
I guess I don't really know his bad stages... for the longest time, I actually couldn't play on Battlefield of all stages, I was really averse to playing there, and if you picked that stage against me, it was basically a free game for you. After a few all nighters playing on Battlefield only, though, that's far from the case, now. Lylat is pretty crazy for everybody so I won't say that. That leaves Town and City, Duck Hunt, Halberd and Final Destination, pretty much, all stages I'm pretty neutral towards.
Some of my best and some of my worst matches happened at Duck Hunt and Halberd (Halberd used to be one of my best stages, until I started training how to play on Battlefield, and now suddenly I can't play on Halberd to save my life), so sometimes I'm a bit scared to go to those, I'd rather not take the gamble.