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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Definitely 343 Guilty Spark, Yes Man, or Nolandroid. That shit would NEVER get old, a whimsical little dude just following me around? Hell yeah.