Which gamepad would be best for SMB?

Play with whatever peripheral you're the best with (be it keyboard, pad, arcade stick).
It is possible to play equally great at SMB using a keyboard, a pad or an arcade stick.
Do not think a specific controller will allow you to play the game better, as it is most probably wrong thinking.

That being said, if you absolutely want to use a gamepad, I would recommend you against picking the Xbox 360 pad.
It is an excellent joypad overall but the d-pad is simply bad.
For games with no actual analog support like SMB, joypads like the Sega Saturn pad (USB version or Saturn version + adapter) or Mad Catz Fightpad are better choices.