I think Joker is partly because fatalities are especially suitable for him, and they've never actually had a chance to give him proper fatalities. (Although his MK Vs DC fatalities were better than most of the others.
Kind of depends on the continuity.He is a brutal, sadistic clown, after all.
Hmm... Ivy wouldn't be bad either. Is she (still) human? Wonder how that could work in lore...