I have a problem in that I've only seen three of these movies. I saw Raiders so long ago I don't remember it, apart from the cool Nazi zapping bit at the climax. I've also watched the LOTR cycle and the Harry Potter movies, but I've also read the books. I loved the books. I didn't enjoy the movies nearly as much, for the usual reasons --- Peter Jackson stuffed around with LOTR way too much and the Harry Potter movies missed a lot of the background and subtlety that made the book series so good. That's not intended as criticism --- I just don't think any of the Harry Potter books can be condensed into < 120 minutes.
If I hadn't read the books I would have to vote for The Two Towers. That was awesome.