In my opinion, none of the LBT sequels will ever outmatch the original. It had so much passion into it that seems to be kind of lacking in the sequels.
Any case here are my pics:
Terminator II - Such a twist to have the terminator as the hero and it worked beautifully!
Dark Knight - Batman Begins kind of dragged in the beginning, Dark Knight was perfect!
Saw II, III, VI - Saw IV and V were good don't get me wrong, but they weren't better than the original.
Transformers II - I might be the only person who liked this movie at all. I was so surprised the critics gave it bad reviews.
Lord of the Rings Two Towers - probably my favorite out of the entire trilogy.
Those are the only ones I can think of at the moment.