I guess these are die-hard comic book aficionados. Maybe I'm not that kind of fan, but I like them all (almost) equally. If my favorite is Iron Man it's because of RDJ... As characters, Hulk is probably the most awkward of the bunch but after the film, I do like Banner quite a lot now. Mark Ruffalo managed to get rid of Hulk's cheesiness imo, which is commendable I also love how Tom Hiddleston portrays Loki. ...all these first rate actors have a lot of fun on screen and it shows ; that's, I think, why the film is so enjoyable, but it couldn't have been possible without Whedon's talent.