Quickshot Command Pad (6-Button Pad):

It's my favourite, not out of choice, but through accessibility : I happen to get only this with my "pocket money" those days. And liked it from the start.
Later when I got my Super Street Fighter II Turbo - DOS CD game, I remember all buttons were used properly. I remember this because, I assigned each button like the Sega Genesis' default button configuration for SF2:SCE (Special Champon Edition), i.e. A=LP, B=MP, X=HP, C=LK, D=MK, Y=HK (where P=Punch, K=Kick, L,M,H=Low,Mid,High respectively).
I also remember all buttons were working in Mortal Kombat 3 - Dos CD game.
Can either choose 6-Button mode or 4 + 2 Turbo/Auto-Fire button mode.