I'm with quite a few people here, anything involving burning, acid, drowning or eyes is pretty much unwatchable for me, yet, give me something like Braindead /Deadalive, where there's literally a scene where zombies tear a guy's entire ribcage out of his chest, I'm fine with that.
I think its long drawn out deaths that freak me out, anything that's a quick death I'm fine with.
That big water tank in Untraceable, along with the heat lamps, it was a great idea for a horror movie, and thoroughly believable, it being based on the theory that people on the internet are a shower of selfish evil fuckers, lol.
Also the one in Cube Zero where there's a dehydrated guy who finally finds water... only it wasn't , it had something in it that broke down the skin, and you see it slowly sliding off his face and arms etc....then you see the bottom of his trousers where it's just a river running down them... nasty.
Oh and just because its a personal phobia, I'd have to go with the inwardly moving walls in saw 5 I think, I've seen crushings before , but never so well done, and the details of bones snapping and being forced out thru the skin etc, just makes it tough viewing.