Druid/warlock was the first dual box I ran 3 years ago. I still think that was pretty fun. Paladin/warrior is pretty fun, though actually I run paladin/warrior/hunter. Anyway, almost anything will two box well, a lot depends on whether you like to be at range, or be right in there. Given the way content is balanced, a two box really runs against solo content with the ability to take the group quests, but you can't instance run by yourself.