Krschkr: In terms of gameplay: Guild Wars.
candesco: 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.
It's a CORPG. There's no open world with other people interfering. Before you enter an area to do quests you form a team that stays with you until you reach the next outpost (a place where you form teams and can meet other people) or return to the previous one. Instead of players you can bring heroes, which are AI controlled party members. However, you can control where they go, you can manipulate the way they use their skills, you decide which equipment they have and you decide which skills they use. Usually you have teams of 8 characters and the standard thing for me to do is to run with 7 AI controlled characters, because they're a blast to play with. They're nicer to play with than players, but apart from that, they allow for huge freedom in combining skills to make crazy team builds. Players would never agree to play things they don't know.
The closest thing to GW's hero system is probably Dragon Age Origin's companion system if you mod it so your companions use most abilities very well on their own and you make them 100% respecable at any time you're not in combat. And if you add 1000 more skills to pick from....
I've never seen AI companions which are, as I said in terms of gameplay, as convenient and effective as in Guild Wars.