Not films I cannot watch easily, but films (or scenes from) that I cannot watch at all:

The end of Ken Russell's The Devils, where Oliver Reed gets burned to death and you see every revolting detail second by second. I had no problem with the sexual hysteria and sacrilege and general weirdness, but that shot-in-closeup execution-at-the-stake scene still sickens me, even in memory. (The fact that the real-life Father Grandier suffered far worse tortures doesn't do wonders for my mind either.) Superbly made, stunningly acted, first rate theatre-of-shock - and I'm never ever going NEAR the damned thing again.   

Dustin Hoffman strapped down into that dentist's chair runs it a close second.