Iron Man 2

The problem with many sequels is that they are swamped by the urge to be bigger and better than the original. Mostly this merely equates to being louder, more frenetic and less coherent. Very rarely do they end up better (The Dark Knight is a notable exception) Iron Man 2 isn't necessarily better than its predecessor but it does remember what made the first one such a wonderful, enjoyable surprise: RDJ's mercurial, witty performance; a focus on the human amid all the metallic suits; and exciting action set pieces. Stark is now juggling the demands of being a superhero, fending off the unwelcome attention of the government, running a multinational corporation and battling with his own mortality (ironically the thing that keeps him alive is also the thing that is killing him) He's more snarky and reckless than ever, unwilling to recognize that he's alienating the people who care about him.
As in The Dark Knight, the law of unintended consequences comes into play. Venal business rival Justin Hammer aims to produce his own version of the Iron Man suit to sell to the military, despite an inherent incompetence (there's a running joke about the essential shoddiness of Hammer's military hardware). Far more dangerous is Ivan Venko, a man with a vendetta against the Stark family. A tech genius to rival Stark, he's also far more cunning than Hammer who foolishly thinks he can control Venko purely because of his wealth. The set piece at the Monaco Grand Prix sets up key character beats (Stark's bravado and vulnerability, Pepper's loyalty) and establishes the very real threat posed by Venko. Seen to be fallible, Stark's world teeters on the brink of collapse: his enemies circle, his behaviour becomes increasingly erreatic and he drives away his closest allies Pepper (even as he realizes he loves her) and Rhodey (who makes off with one of the suits after a bout of robotic fisticuffs that wrecks Stark's malibu mansion)
If there's one problem with the film it's the need to exist on 2 separate levels - on its own terms and as a piece of the Avengers jigsaw puzzle. Cue Nick Fury appearing to lecture Stark and provide exposition (the file helpfully entitled Avengers Initiative is hilariously heavy-handed); Black Widow infiltrating Stark Industries (and in a great scene taking out ALL of Hammer's security while Happy's struggling to deal with one guard); and Stark's resourceful yet amusingly disrespectful use of Cap's shield. As for the Iron Man films themselves, there remains just a small, lurking suspicion that too much action involving men in iron suits and/or robots hitting merry hell out of each other might ultimately become just a tad repetitive - though Jon Favreau (unlike Michael Bay) thankfully seems to understand that the action works as well as it does because the audience feels empathy with the characters. The Iron Man suit is indeed incredibly cool but it would count for little without the fascinatingly flawed Tony Stark inside it.