Phantasm 2 easily wins over the original, but I still love the first one. It's not that the first is bad, its just that the second is that great.
Terminator 2 easily beats out the original in my opinion. Awesome epic movie. No contest for me.
NOES 3 is a heck of a sequel, but the original is just such a classic that I still place it at the top of the series.
And to shake things up a little bit, I'll throw in Jason Takes Manhattan. Yes, it is better than the original and seriously under-rated.