My least favorite would have to be The Incredible Hulk, but I'll also admit: it's my top personal-favorite solo Hulk film, since it smashes the crap out of Eric Bana's excuse of a film from '03.
I love them all and don't tend to focus on least favourites. If I had to say, put them in order and look at what would be at the bottom, it would probably be Iron Man 3.