Who would it be? I would personally pick Wheatley from Portal 2. While kinda a screw-up, he's frickin hilarious.
Edit: Just to clarify, I meant like an artificial intelligence, a machine.
Edit: Cortana seems to be the favorite here (no surprise).
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Ghosts from destiny. Revive you if you die, warn you of dangers, hack everything, tell you cool facts, analyze something for you, scan an area. Anything! Ghosts are the best