Neverwinter Nights 2 for me. I always use a party with Neeshka, Elanee and Qara. Neeshka for removing traps, unlocking and pickpocket, Qara for having a pure spellcaster and Elanee can serve as healer or as support. Khelgar is also a good one, a pure tank. Does his job right and when you give him the gloves of ironfist he even packs more punch. I also like that the companions in NWN 2 do have a personality and each has it's own story. The game might have a history of being plagued with bugs and how it got fixed several times, but Obsidian did a great job and especially the companions here are fleshed out. I have that less with the first neverwinter nights or with the dark eye: drakensang.
Krschkr: In terms of gameplay: Guild Wars.
Guild Wars is a MMO, which means chars are played by real people. Only the npc's in there are computer controlled. There are no AI companions in there. So that doesn't count.