My pick would be Lord of the Rings.
X-men - it was alright, I just didn't quite like how things ended up in the third one. This movie would be good if you have no clue what X-men was prior to the release of the first movie.
Jurassic Park - Don't quite remember the second one. The only thing that was enjoyable about Jurassic Park was getting to see the dinosaurs eat people.
The Matrix - First one was a classic. Second one had cool fight scenes but that's about it. Third one, some cool graphics except for when Neo and Mr Smith have their final combat. It looked so cheezy. The last two, especially the last one, had bad storylines though.
Austin Powers - Funny, great movies. Still not as memorable as LotR.
Mission Impossible - Didn't quite like the second one. First and third ones were good I guess.
Blade - Only saw like the first one or the second one, I can't remember. But from watching that movie, this movie really didn't seem that great to me.
Saw - I only watched the third one a few weeks ago. I was told about the previous two. Not quite fond of what happens in the saw series though.
Terminator - First two were great. Third one... only thing good about it was that the bad terminator was a hot chick
The Fast and the Furious - I don't think you can call this a trilogy since they really have nothing to do with each other, particularly the last one. But also, I haven't seen the last one.