3D without a doubt. 3D Mario >>>>>>>>>> 2D Mario.

It depends for me.  I'd take Super Mario 3/World over Mario 64 any day of the week.  I think what you grew up with is a major factor too.  For people who started with the N64, Mario 64 is likely their favourite.  I started with the NES.

But Mario Galaxy pretty much destroys them all, in my opinion.

I grew up with 64 and it's one of my least favorites. Galaxy is excellent though. I just generally dislike all of the 2D ones.

Argh.. Choosing between 2D and 3D platformers is like choosing between a really hot brunette and a really hot blonde. I personally can't choose between the two,  simply because there are many awesome 3D platformers  such as Mario Galaxy, Banjo & Kazooie, Crash Bandicoot 1-3, but there are also many great 2D platformers like the original Sonic trilogy, Mario World, Mega Man 2, Symphony of the Night.. So yeah.. Can't choose between the two!

2D.

A lot of 3D platformers have issues that 2D platformers don't normally have. Most are fetch quest in design. Most are less platforming than they are exploration, Simplistic gameplay with less variety in a 2D game works better than simplistic gameplay with less variety in a 3D game.  3D platformers tend to have less of a penalty for missing a jump.

Not counting games that are more action/adventure than 3D platformers, games like Shadow of the Colossus, Ys Oath in Felghana, etc, my favorite 3D platformers are Gurumin, the PSX Crash Bandicoot games,  Super Mario 3D Land, Mirror's Edge, and the first Jak and Daxter.  Not sure if Mirror's Edge would be a 3D platformer or not, but it's mostly running and platforming, so might as well.

currently playing Rayman3 HD and Rayman Origins, both good for different reasons. Its all good

I agree. From the N64 collect-a-thons (SM64, Banjo) to PS1 Spyro, Crash, Ape Escape. That gen had the best collection of 3D platformers, nowadays 3D pure platformers like Galaxy which I liked, are few and far between.

Ratchet & Clank is good too, though it has more emphasis on weapons than platforming.

2D by far. Enjoy both arcade and puzzle platformers. Don't really like 3D platformers at all.

love both

Both. I adore Crash 2, absolutely love it beyond words. Recently I played Jak and Daxter, which was near platforming perfection (not sure what happened to 2 and 3, racing games are not platformers). Then I played Guacamelee which was nearly as awesome....