2D Mario is far far superior to 3D Mario from a gameplay standpoint. The only thing 3D Mario excels in is asthetics and soundtrack.

I literally cannot play 2D mario. It doesn't give me anything...

I however love the visuals of the 3D games and exploring world and playing with the new gameplay mechanics.

Agreed.

@OP: For me it's pretty simple. From 1985-1995 it's 2D Mario. From 1996 onward it's 3D Mario. As a unit, the 3D Mario platform games have been more impressive than the 2D Mario platform games since Super Mario 64.

Overall, however, I would award first place to 2D Mario. But if EAD continues to make 3D games like Super Mario Galaxy, the scale will tip in the opposite direction.

Easily 3D.

2D Mario can be a bit easy and repetitive, in general 3D is more interesting in my opinion :)

Probably 2D Mario. Comparing Galaxy and what I've seen of older 3D Marios to the awesomeness of NSMB DS/Wii definitely puts 2D way over.

Somehow I really, really like 3D Land, though, while NSMB2 was pretty meh. Although, that's not nearly enough to counteract how much I hated Galaxy and loved NSMBW.