Crysis' Cryengine is the best of all FPS genres, and he really needs someone to make Bots for him. I am sure that this is C ++, and if you can then, you really will go very far and be in demand. it's free
Either this or the UDK - Unreal Developer Kit - free - OOP structured engine I think (sorry I'm not a coder - just an artist and a writer. :)
Good luck.
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