White pads - original, no Turbo (!!!) switches, buttons I and II only
Grey pads - color matched to SuperGrafx and CoreGrafx systems, had blue writing/markings, Turbo, I and II
Black pad (TG) - the same as the Grey pads with black plastic and writing coloration that matched the TurboGrafx-16, large DIN plug
Black pad (Duo) - same as above with white "TurboDuo" logo and mini DIN plug
Avenue Pad 3 - buttons I-III
Avenue Pad 6 - buttons I-VI
I think that's about it for the NEC-released pads, not counting the joysticks.
Personally, I like the Avenue Pad 6 for most all of my gaming needs. The pad is larger and has a better design, IMHO, than the smaller pads. Larger (to a point) always gets points in my book. I like how the Turbo switches are on the opposite side so I can use my left thumb to toggle them without having to take my right off the button. If I can't use my Avenue 6, then it really doesn't matter which one I use, since they all have the same physical dimensions. Although if I'm playing a shooter it better not be a white pad.