Rogue, warlock or another frost mage as I see it.
Going with a rogue is actually very fun, and high ratings aren't too tough to get as long as you communicate with each other. The same goes for a warlock except that where the rogue brings massive singletarget dmg, the ability to stun an opponent to death and a blind, the warlock brings a healthstone, fear and a pet which can really mess up a healer.
The duel frost mage teams are harder to get high ratings with (2k+), it can be done, but this setup requires much better timing than the 2 mentioned above. While being a lot more challenging it's also a lot of fun, having 4 elementals, the amount of shatter combos you'll be able to do are really frighting as is the damage. The biggest problem of this team is LOS abusing healers - AKA druids, so don't expect to rise much above 2k with it.