I really hated the Sega Genesis controller. The control pad and buttons just didn't have a good feel to them. The shape also didn't seem natural to me.

my experience thus far...

atari 2600 - hated (big stick with a little base)
atari 5200 - didn't hate nearly as much as everyone else, but was certainly unpredictable
NES - liked once i got used to it, but those sharp edges on the joypad...ouch.
SNES - pretty good, but many of the popular games at the time needed 6 buttons (SF, MK...)
Genesis - no complaints here
Saturn - odd looking and awkward
PS - perfect digital controller (when analog came the location was awkward)
N64 - love it (probably my favorite so far)
Dreamcast - nice layout, but too small, angled wierd, and cord in back?! my wrist hurts just thinking about it.
PS2 - see PS1 (easily the most functional yet)
GC - Not much experience, but it feels pretty good to me
X-box - too big, too heavy, too ugly...seems functional though

I second the vote on the Genesis controller, didn't it have like 3 buttons righ in a row some reason. Like A B C

Also, I wasn't the biggest fan of the NES controllers and also the Ascii controller with the little circle controller thingie. I remember getting both the power grips, and the little joystick things you could buy and clip on to the top of the pads to make them somewhat acceptable to me. However, in the end I only found happiness using the Advantage.