Really Shaman or Cleric (or even a Druid, to a lesser extent) plus any melee/hybrid will work decently well. Likewise a pet class and almost anything works great (because pets are beefy and adding a second player removes the 50% XP penalty when the pet out-damages the player).
But in particular ...
- Shaman/Monk (monk tanks and shaman slows and heals, so you essentially have the core of a group with the smallest number of people possible)
- Warrior (or knights)/Cleric (not the absolute best duo, but great if you like grouping: grab an Enchanter/Shaman and any DPS and you're good to go)
- Enchanter/Cleric (also good for grouping, but can do some amazing duo stuff too with the cleric healing charmed pets)