Wow, the more I'm thinking about this, the tougher it is.
Right now, I really enjoy SMW more than SMB3. But, that is really because I burned myself out on SMB3 when it was released. I only had 3 NES games for the first year I owned the system. So, I would play SMB3 all the time. Also, when friends came over, that's what they wanted to play, so we played that.
After that year, I find it really hard to play the game now. I just feel done with it.
I never had a SNES during it's run, so I only played a little bit of World. So, even though I've played it quite a bit, no where near as much as 3.
Though, if I'm being completely honest, my favorite 2D mario game right now is Yoshi's Island. Yes, baby Mario is annoying, and the game seemed a little too cheery, but it felt really deep and I loved the music.
I think I'll pass on voting right now.