In the old days (TBC) I did a prot warrior and resto shaman 1-70 together.
This was not the most fun.
If I ever did a "practical" group like that again, I'd change the tank to a druid for the extra buffs and stuff.

Sorry, whenever I think of "funnest" it ends up turning into logical or pragmatic. So what's the funnest?

For me that'd be hunter/warlock. 4boxing for the cost of 2boxing!