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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Two choices come to mind. Cl4P-TR4P because I can let out my anger on him and he's funny. Eve from Mass Effect 3. Hot robot body! ;)