Dead Space 1 and 2 struck me as being the goriest. Fallout 3 and Fallout: New Vegas don't even come to mind, in comparison. Left for Dead 2 was also surprisingly gory, though in a different way than Dead Space II.
My reasoning would be a combination of things. Outright goriness where people have seemingly had their skin flayed off while they're still alive. The mutations of the necromorphs themselves can be very twisted and discomforting looking. The corruption and the "Guardians" that grow out of it use their intestines to lop off your head, and more immature versions of the same guardians scream for help, despite being disembodies and glued to a wall (although mature ones do that too to a lesser extent).
There would also be the matter of the only surviving crew members mutilating themselves and each other to death. Or random crew members sobbing or beating their head against the wall. And that guy who got sucked up into the vents and was spat back out in chunks. It was a game that, at least I found, did a very good job of being very, very gory without the overuse of blood.
The death scenes also, of course, come to mind. The disembodied head that takes over your body was always my favorite! Hope they have it in the third game too!
I suppose a person needs to keep in mind that "gore" essentially equates to how well you can see the exposed bones and organs of characters in a given game, not how much blood there is. Or that's how I always figured it worked. "Blood and gore". Then you have people who are so conservative and sensitive that they see violence, blood, and gore as a combined blur of disgust that unsettles them.
It felt really good to cut out that centaurs guts in God of War III, I gotta say, though. But God of War III still doesn't surpass Fallout or L4D II in a lot of ways.