Cool thread !
Okay, here goes
1) the Guardians of the Galaxy: until the film I didn't give a rat's ass about them. I remember Gamorra and Thanos, because I was a fan of Adam Warlock back in the 70's, but since then I didn't care about them at all.
At a glance I was pretty skeptical about a talking raccoon and a walking tree, but the GOTG film made me love them. I have an old Star Lord comic, drawn by John Byrne and written by Chris Claremont (possibly one of the greatest writer/artist teams ever) and Star Lord is still boring - the film made him cool.
Hell, I hated Yondu in the comics, he looked like a blue rooster, but the movie version was awesome ! I loved Michael Rooker as Yondu, he could carry a movie to himself.
2) Thor: I enjoyed Thor during the 80's Simonson run, but went off him quickly after that. Didn't like him much in the first Thor movie, and in the Avengers films he's pretty much a bit of moving furniture who has the occasional line of dialogue, but in Dark World, I found him a lot more likeable and funny - and while Hiddleston is great as Loki, in Dark World he doesn't completely steal every scene he's in with Hemsworth (which is saying a lot, as he pretty much steals every other scene he's in).
3) Captain America. I HATE Captain America, I have always despised the character, but Winter Soldier was such a good film that I could actually sit through a whole Captain America movie. Chris Evans made Cap tough but likeable-ish.
4) Nightcrawler: Found him kind of lame in the comics, but in X2 (and this is saying a lot, as I'm NOT a fan of Bryan Singer's work), he steals the movie with his attack on the White House.
5) General Zod: Pretty much thought he was just a dick, but Shannon's portrayal and Goyer/NOlan's characterization made him a much better character.
6) Ra's Al Ghul: vastly preferred Nolan's version, makes a lot more sesne.
7) Hawkeye: kind of liked him as a comic book character, really liked the movie version.
Just a thought dude, you could do another thread for "movie versions of comic book characters that you hated!" for movie versions of characters that made them less likeable.