Well, for me, it goes down to The Suffering and it's sequel, Ties that Bind. I find going on an all-out adrenaline rush can be deadly, tear enemies (and people) into bits and all that jazz.

The others I find only midly gory like RE4, F.E.A.R, Spartan: Total Warrior, Painkiller which are very very amusing.

I'd have to say God of War though other games like Turok 2, Mortal Kombat II, Resident Evil 4, Sword of the Beserk: Gut's Rage, and others get honorable mention.

I think God of War is my favorite. I never get tired of grabbing an enemy and ripping him in two, accompanied by a battle cry from the hero. Ah, the felling.