This was hard ! I'll go in order from fav to least fav
1. Rocket....just has the best lines, and we see a huge range from him, despite the fact that he's a CGI raccoon. We see him angry, crazy, sarcastic, wickedly funny and then tearful. I never thought I would care about at CGI raccoon before GOTG, but Rocket was fantastic.
He's a crazy little critter (especially when he and Groot take on the Sentry goons) who takes no crap.
2. Star Lord, in the tradition of Han Solo and Malcolm Reynolds, he's another outlaw, but instead of pretending to be a big badass, he doesn't take himself seriously at all. Tremendous fun, he had me at the dance number in the opening sequence !
3. Yondu, a small role but a memorable one, and a much cooler version than the comic book Yondu, this one had some balls and menace, but also lots of humour.
4. Drax: Bautista actually has really good comic timing.
Rocket Raccoon: [about Drax] Metaphors go over his head.
Drax the Destroyer: NOTHING goes over my head!... My reflexes are too fast, I would catch it.
Peter Quill: We've already established that you destroying the ship I'm on is not saving me!
Drax the Destroyer: When did we establish that?
Peter Quill: Like three seconds ago!
Drax the Destroyer: Well I wasn't listening then, I was thinking of something else...
Negbula: Gamora, you've always been weak! You stupid, traitorous...
[Drax blasts Nebula]
Drax the Destroyer: No one talks to my friends like that.
LOL !
5. Groot: Diesell does a lot with variations on three words, and the animators are just superb.
6. Ronan: a real B-list Marvel Villain who comes across as a credible menace and is genuinely scary as a genocidal fanatic. Well done Lee Pace, a long way since Pushing Daisies.
7. Gamora: yeah, kind of a mix of Uhura + Neytiri, Saldana looks incredible (even green) but is probably the least interesting of the cast. Still good but not even close to Rocket.
That's just how I see it.