Best duo?

[Ryba]

Splorf22 said it: skill matters way more than classes. How much fun you have with your class is a better predictor of your eventual skill with that class than that class' theoretical maximum efficiency. So, as long as your duo involves a class that has some way to heal itself and some way to snare or slow, you can accomplish a lot.

If you want to kill dungeon targets you shouldn't be able to kill, part of the duo should be an enchanter, which would mean the other half should probably be a priest class.

If someone in the duo wants to play a melee, they should roll a monk because nothing else will have the same level of manaless, front-on dps and survivability. This doesn't leave much room for the other player to be anything but a shaman.

There are specific circumstances where other combinations will be "better," but generally speaking, everything else is less awesome.