Magneto.
Tony gets his butt kicked in the first encounter, maybe, but then he just develops a non-magnetic alloy (it’s not that hard, even for non-Starks), builds a new suit out of it, and the rematch is trivially easy. Heck, his current suit (supposedly a gold-titanium alloy) probably isn’t magnetic to begin with.
It’s not like Wolverine, who’s intimately bonded to his skeleton. If Iron Man’s suit is a liability, he can just build a new one that isn’t.
Oh happy day! This thread has been bumped and I didn’t have to do it myself.
So my personal thoughts: not as solid and coherent a film as the first movie, but more fun than the first. The first film has that long portion where Tony is building the suit in captivity in Afghanistan, which is pretty intense and epic, but ultimately it’s a lot more fun to watch Tony be a rich brat. Plus there is lots of gratuitous action, like the racetrack scene and Black Widow beating up all the guards in Hammer Industries (I love Scarlett Johansson in this! She’s a triple agent, a computer hacker, and a kickass martial artist - I’d like to echo Tony’s sentiment of “I want one”). And it’s a summer superhero movie so I say ‘gratuitous’ as a compliment.
I also thought Ivan Vanko was a really interesting character. There’s an ongoing theme of ‘legacy’ throughout the movies (emphasized very explicitly in the beginning of IM2) and I like how Ivan represents his father’s legacy. He’s also Howard Stark’s legacy - a legacy of having screwed people over. Mickey Rourke gave Vanko a lot of depth; the bird and the prison tattoos were his additions and I think they make him more human. I also loved the parallel between Tony in Afghanistan and Ivan in Hammer Industries - both of them are told to build a weapon but are secretly building suits for themselves. While Tony and Ivan may have had completely opposite lives, and are working against each other, they are actually very similar in the way they think!
Unfortunately Ivan’s arc got a pretty lame resolution. I think someone else on this thread commented that everything is too easy for Tony in this film, and that was basically my feeling too - there are never any really tense moments, while Movie 1 was full of them. Luckily Robert Downey Jr. plays this character so well that every moment he was on screen was great even if he was doing something stupid from a plot perspective.
By the way, has anyone got translations of the Russian bits yet? I only understood him saying “Fuck you, Stark, fuck you” on the racetrack.
I don’t know, when he doubled the frequency like Tony suggested, his arc worked pretty well.
For this comment, I internet-bake you a whole rack of chocolate chip internet-cookies. 
That’s another point. Stark said something in the movie about her being a “triple imposter”. I’m not seeing that. She was a SHIELD agent pretending to be a Stark Industries employee - that’s pretty much a one-level imposter. How was she a double imposter much less a triple one?
She pretended she hated Pepper when around Tony so the two of them could better plan against him. I assume she was also finding out stuff about SHIELD for Stark Industries?
…but you’re right, it is pretty dubious. So I’ll amend my statement to say “she is a secret agent” instead.
Magneto’s powers don’t work that simply. While they are strongest on ferromagnetic metals, they do work on all kinds of magnetic materials, including para- and dia-magnetic materials. Gold, for instance is very strongly diamagnetic. So’s carbon. Water, too. See, for instance, how he manipulates the diamagnetic copper skin of the Statue of Liberty in the first X-men movie, and he stops bullets all the time - bullets usually aren’t made of very ferro-magnetic materials, are they?
Tony would be better-off with some sort of dampening field. Or a plastic exoskeleton.
Magneto would be interesting. Or maybe have a villain who can become intangible (like the X-Men’s Shadowcat or the ghost brothers in Matrix 2). Stark would have to figure out a way to fight somebody who’s immune to being punched or hit by repulsar beams.
Eh, the point is, he’s adaptable, and can develop ways around his weaknesses. Or at least around the weakness of his suit: His own personal weaknesses are more difficult.
Is there any villain who works by magnifying his victims’ self-destructive or addictive urges? That’d be a good foil for Tony Stark.
Tony Stark vs. those plants from The Happening?
I think Stark’s put a lot of his self-esteem into being Iron Man. Neutralize Iron Man and Tony Stark loses that mental crutch and his self-destruction will follow.
A friend of mine said that Iron Man’s arch nemesis is Jack Daniels.