Classic Controller Pad: almost always
Tap-Jump: off
D-Pad: taunts
L: shield
R: attack
Both Zs: unused (shield)
a: jump
b: special
x: grab
y: unused (grab)
C-Stick: smash attack
Attacking and teching have to be quick, so the shoulder buttons are used. Zs and y are two far away, so I ignore them. Jumping and specials are used a little more than grabs, so they're closer to my thumb. C-Stick is kept at smashing for techniques like hyphen smashing. No tap-jumping also makes u-tilts easier.
For GameCube controllers: Almost as good as the classic pad, but not so comfortable
Z: attack
L: shield
R: unused (shield)
A: special
B: unused (special)
Y: grab
X: jump
D-Pad: taunts
C-Stick: smash attack
Tap-Jump: off
Same reasons, just reorganized.
Wiimote and Nun chuck: only if it's my only option
Tap-Jump: off
Shake Smash: off
Z: jump
C: shield
A: special
B: attack
A+B: unused (grab)
D-Pad Down: grab
D-Pad Side: side smash
D-Pad up: up smash
Numbers: taunts
Same reasons again, but jumping has been moved to the left side to make the d-pad feel more like a c-stick. Also, I'd rather not use two buttons at once in case I only hit one of them.
Wiimote: Only against a n00b.
D-Pad Up: jump
B: shield
A: grab
A+B: unused (Grab)
-: taunt
1: special
2: attack
1+2: unused (grab)
Tap-jump is the only good jump button here. Funning grabs are basically non-existent, so it won't matter if I'm holding a direction while grabbing. Regular attacks are used more than specials, so they're closer to the thumb. Lastly, I'd rather be playing my best with a horrible set-up than holding back with a better set-up.
Guitar Hero guitar: would'ave been sso awesome.