For me, very few horror movies in the 'gore' niche are actually that disturbing, especially if you throw in a creature to the feature.
Violence however, can be pretty grim. Irreversible contains some of the worst scenes I have ever seen. Hostel was pretty uncomfortable to watch, reminded me a bit of the Saw movies.
Of all those slasher/gore flicks a la Wrong Turn or Texas Chainsaw Massacre, I think Wolf Creek is pretty good. The main problem with this type of movie is that you have little time to care about the characters... in many you kind of want them to get killed! With Wolf Creek, it starts out like a good old fashioned road movie, plenty of time to empathize with the leads before they start getting their bits snipped off!
I though the Descent was brilliant, especially for a creature feature. They managed to capture the essence of claustraphobia before even showing you a monster; and even after revealing the creepy crawlies, the gore and violence keep you on the edge of your seat!