Alright, so having recently gotten back into FPS games I'm in the mood to try more of them. One problem however is that it seems that most FPSs lack challenging final bosses or even final bosses at all sometimes. The Half-Life series, Halo, and Call of Duty, while I've had fun with them all, they fail to give me a challenging final opponent. So fellow escapists, are there any FPSs with good final bosses that you would say are worth playing?