Paladin or Shaman are the best choices for duo with warrior for obvious reasons. However, even with a pocket healer, warriors aren't a great solo/duo class outside of just leveling or trivial stuff like farming mats.

If you want to solo end-game stuff and instances learn how to play a hunter or mage well. I think hunters have the highest potential with all the goofy changes Blizzard made to kiting in instances.