Not the best Marvel film (I'd rank it fourth, personally, though that's provisional), but definitely the most unique. Partly that's the setting, partly the story type, and partly the approach of the director -- I find it kind of interesting to compare the almost universally positive reception for this movie, which is an outright comedy much of the time, with the controversy around the jokier parts of Iron Man 3 and Thor: The Dark World.
I've been a fan of Chris Pratt since Everwood, so I got a kick of seeing him become a leading man in a good blockbuster. He's great as Star-Lord, handling both the comedy and the more serious elements.
Zoe Saldana was good as Gamora, though I think the writing for the character could have been improved slightly. In theory there's a lot to the character, given her relationships with Thanos and Nebula, but those don't quite have the weight it feels like they should. Anyway, they can remedy that in the sequel, and I liked that, unlike a lot of "strong female characters", Gamora gets her own share of the jokes.
Bautista was fine as Drax. Cooper was really good as the voice of Rocket, who is the new gold standard for bringing a difficult comic book character to the big screen. As far as villains go, Ronan was okay; Nebula, as I already said, was underused. I liked Thanos' appearance, though Brolin's voice isn't quite what I imagined for him.
In amidst all the comedy, there's a couple of moments of unexpected beauty that really stood out, like the first time Groot deploys those light-spores in the darkness. That felt genuinely a little magical.