So, I think Arnold makes for a better villain, despite Robert Patrick’s best efforts.
But, being the kind of person who thinks Terminator 1 is better than Terminator 2, I honestly don’t think anybody could out-Arnold Arnold.
You could argue that the Predator is Arnold’s best antagonist, but again, I would say that the Predator went on to much bigger things, and he even overshadowed Arnold.
Ask anybody, and they’ll probably say that the best Arnold villain is the T-1000.