Haidilao (Or Hi! as the sign says) is a safe bet, and there are several 24hrs one's (or near-24hrs, since they close between 5am and 7am or so..)

Little Sheep is also very good - if I want a "smooth" hot pot experience, it's always either one of them (Both have surprisingly nice choice of beers at fair prices, too - something that is otherwise quite lacking in China)