Are we talking original campaigns and expansions, or are we including mods and multiplayer servers? Not voting until I know, especially since I've never completed the single player portion of any of them (unforgivable I know).
Original campaign: Icewind Dale 2
Multiplayer: Battle of the Dragons - Neverwinter Nights, expansions optional but recommended for prestige classes (Team PvP/PvE, you join forces of one of two dragons Aniel, a blue dragon, or Roshnak, who I think is a copper dragon despite having acid breath. You and your teammates fight the opposing team's troops (short sword wielding warriors and archers early in, long sword wielding warriors and mages toward the end), defenders (powerful stationary archers), elite defenders (super powered defenders that normally take multiple people to kill), the other teams players, and finally the other teams dragon. Whoever kills the enemy's dragon first wins.)