Mario: both, but I prefered 3D
Zelda: both, tie
Metroid: 2D, I don't consider 3D a Metroid

Mario = 2D
Zelda = 3D
Metroid = 3D

What azzer100 and Words of Wisdom said. 2d, 3d, lo-res, hi-res, dolby pro logic 2 or mono, Mario, Zelda and Metroid are what defines gaming for me. Not because I'm a Nintendo fanboy, but because I'm a MarioZeldaMetroid fanboy.

When it comes to gaming, these franchises are the absolute top.

mario= 2d
Zelda= 3d
Metroid= 2d

for all of them both 2D and 3D are absolultely amazing. but i'm gonna go
mario-tie (all marios are incredible)
zelda-3D (going back to play a link to the past now it isn't even close to OoT)
metroid-maybe 2D cuz it was more action while the 3D ones are amazing and gorgeous but i think there is too much scanning and reading and not enough action.

Both...

Mario: after Mario64 and playing SMG i know that 3d is where Mario belongs

Zelda: Both. i love Zelda on the N64 and on the wii, but i also love it on the gameboy.

Metroid: 2d. I loved Super metroid and Fusion. I also like Metoid Prime1, but after all these years i haven't beat Echoes even though i had to restart that game so many times. I just recently restarted it again a few weeks ago and i can honestly say that i don't like that game. I get about 1/3 of the way done with it every time and then lose interest in it.

3D for all.

i prefer them all in 2d.

Mario - 2d
Zelda - 3d
Metroid - 2d

I would absolutely love a new 2d Metroid in the spirit of Super Metroid on console. Put it on WiiWare or something, none of you know how happy that would make me.