I asked myself this question recently, and decided to make a top 10 video for it. I prefer the 3D platformers over the side-scroller of the last decade, but am crazy about the classics as Mario 3 and Super Mario World. Here's my list and let me know what are your favorites!

I'm not a huge fan of 2D platformers at all.... Way too many out there right now IMO. But I love 3D platformers, so I'm gonna have to say 3D World.

The best 2D Mario is Super Mario World, and the best 3D one would be Super Mario 64, even though Galaxy isnt far behind.As the best one is hard to say, but I guess I would go with Super Mario World.

Super Mario Bros 3 (especially the Super Mario Advance version) - 2D
Super Mario Galaxy - 3D

The first Galaxy is the most ambitious, original and innovative one. 3D World is the most fun to play.

2D Marios are outclassed by tons of other 2D platformers, so I'm not even talking about those.

I fully agree with your #1.
Thats my favorite too.

Im hopeing Nintendo does a Galaxy 3, for the NX or a new Sunshine.

Super Mario 64. Never found any of the other games to be anything special.

Galaxy. I actually didn't warm up to Mario 64. Galaxy had the charm

Galaxy is the height of Mario.
Its the best mario has ever been, so I hope they do more of them.

Im tired of all the 2D or 2.5D style mario games lately.
I think Donkey Kong Country, does it much better than Mario when it comes to 2D platforming.

Super Mario Galaxy.

It's about as close to perfection as video games get.