Hmmmm, it's between Dead Center and Dark Carnival for me. Although Dark Carnival has playable mini-games, so that might just be the tie-breaker right there.
I'm basically looking forward to Dark Carnival since I've seen a lot of gameplay footage that is quite impressing to me. The Parish campaign would probably be my second choice.
I think Dark Carnival might be the campaign I'm looking forward to the least. I find carnivals and circuses very boring, and I never did get the scare about clowns so they don't do too much for me.
I say the least and not really mean it, as I'll still find it fun. I didn't like No Mercy at first, then loved it. Loved all of them.
Not sure which I'm looking forward to the most really. I've tried to not read up on everything L4D so I won't know whats going to be around the next corner in the game.
Dark carnival sounds awesome and Hard Rain sounds like it will have some nice features too. I'll enjoy them all no doubt.