I always duo with a mage friend of mine. We never die, and always have loads of fun. If we need to do lots of grinding or something he always just does aoe, and I spamheal him. We've been playing together for around 40 lvls now, and I'm very happy with mages! I like mages best- mob is allready dead before I get a chance to heal most of the time ^^
But I guess every class is nice to duo with in the end. I've played with a lot of friends, and they were all nice as a team (warrior, hunter, another priest, warlock, ect)