'Fun' comps? Well, on PVP the fun is 'to kill and survive', so comps should be build based on that... But I don't multibox PVP so, I'm not the right guy to talk about it.

For the fun of it, and generally speaking, I like hunters and mages combos, I mean they're funny to play! 3 of each, plus tank and healer.
DKs are the ultimate power and very funny to play too. 3, 4 or even 5. But here you'll have some particular issues that need additional configuration and additional focus (there are slaves and then there are melees...).

Again, I say: class is not what gives that characteristic any of us is looking for on a multiboxing team. For instance, on this case - funny teams - race, hairstyle, outfit, pet choice, ... can bring a big part of that fun you're looking for.