The Mandarin. Easily.
Not because I love the character or whatever, no, I hate what they did because basically all of Iron Man 3's excellent trailers, posters and tv spots sold us a completely different movie from the one we ended up getting.
About a whole year's worth of spin sold this movie as an epic battle between Hero and Nemesis in which Stark finally suffers defeat and had to fight his way back from destruction. Kingsley's Mandarin looked and sounded friggin' ferocious too... THIS was gonna be good!
Now, studios overhype their films all the time. No surprise there. But what Marvel did went way beyond that... they outright lied
about what the film was.
And even then -even then!- I could have forgiven that, all of it... if what the film was actually
about wasn't so friggin' boring...
Eh, rant over.