Here Comes the Boom is highly underrated. It's a mixed story of underdog and Cinderella as well as a great modern tribute to Rocky. Nice acting, lots of action, plenty of laughs and very interesting dialogs. There are some predictable plot elements, but each one of them is underlined with a certain whimsy. There's also a few fakes that show, however, it definitively doesn't take itself seriously.
For a good time and some good action.