Of the ones on the list, Iron Man, definitely.
Of Marvel movies, Spiderman 1 and 2.
Of superhero movies, Batman Begins.
No offense to those of you who like it, but I didn't like Captain America much at all. Almost no character development, plot was pretty weak, and the main character was...Well, just too Mary-Sue I suppose. He seemed more invincible than Superman, and he was just too "good" at what he did. Nothing ever gave him trouble, he never really got hurt, and the final "Boss fight" was just really...Lame. Just my opinion though.
Hulk was alright, probably as good as a Hulk movie will ever get, but certainly not a favorite.
Thor was the funniest of the bunch, and I liked most of the characters and plot, but there should have been an extra 10-20 minutes in the middle somewhere. More of the whole "Proving his worth" thing. It was pretty much all over in one scene. Also, I doubt a sequel will be very good, because the only reason the first movie was any good was because he didn't have any power for the majority of the movie! *SPOILER ALERT* He gets his power back at the end of the movie *SPOILER ALERT* and I'm not a fan of invincible characters, so I doubt I'll like Thor 2.
Iron Man was pretty good though. It had characters, plot, character development, the main character wasn't the typical goodie goodie hero, the main character wasn't invincible, and the acting was good.
I didn't like Iron Man 2 very much though. It seemed like it couldn't decide whether to try and be dark and gritty, or lighthearted and silly. The main character was just annoying, the plot was meh, and the final battle was pretty dull.
It seems to me like the Marvel movies made in preparation for the Avengers all seem to be a lower quality than the previous ones. Spiderman 1 & 2, the Xmen movies, Hulk, and Iron Man 1 were all better(IMO) than the newest bunch, Thor, Iron Man 2, and Captain America.
Just my opinion though.