There are a lot of movies listed here that I can't believe people are calling UNDERRATED. Movies like Batman '89, which I thought was almost universally loved (at least loved enough to spawn 3 sequels anyway). Others, I agree are VERY underrated, while some aren't underrated at all, they're just awful (like Batman & Robin).
I certainly agree that the following movies are underrated:
The Watchmen
I find this movie very watchable. The casting, acting, costumes, and sets are all very well done. The story itself is very faithful to the source material (except for a couple of relatively minor liberties taken). Sure, the story moves at a pretty slow pace. But guess what? So did the comic! I really don't get all the hate this movie seems to attract.
The Fantastic Four 1 & 2
While not great superhero movies by any means, they are incredibly watchable. Three out of the four leads were very good in their roles. The only one that seemed forced was Jessica Alba. Not that I don't think she's a good actress, because I do, I just think she was all wrong for that particular role. She was simply Hollywood's latest "It Girl". The actor who played Doctor Doom was okay in the role as well, it was just that the character was poorly written. Had they maintained his true origin story I think that the film would've been far better received. But still, the sets, costumes, basic story, acting, etc, was all quite good.
Spiderman 3
I agree it should have been just about The New Goblin and The Sandman, or maybe New Goblin and Venom, but having all three in there was too cluttered. However, the acting, VFX, etc, were all top notch. Just as good as the previous two installments, if not better.
Dare Devil
I actually found this movie to be very well thought out and exciting. I don't really get all the hate that it constantly receives. I even enjoyed the fight in the park between Matt & Elektra (a scene which seems to be the main reason why everyone else seems to hate the movie). I didn't even mind them making The Kingpin black (how many extremely large, strong, bald headed actors do you think are out there?).
I can't really come up with anything else right now. It's after midnight and my eyelids keep trying to close on me. I might add more later.