Gods that's an easy one. Terminator 2 is far superior to the original in just about every way. There's a reason I have T2 on bluray but none of the others on any format. It's like the greatest movie of my childhood.
I think there's some majority opinion that Godfather 2 or something was better than the first, but those movies were way too slow and put me to sleep, so I can't really comment too much on that debate.