Hard to look past The Great Silence I suppose, but I really like him in both If You Meet Sartana, Pray For Your Death and I am Sartana, Your Angel of Death.
Still, apart from maybe For a Few Dollars More in a small part, how batshit crazy is he in the spags? He always seems more crazy-like-a-fox to me, often with an almost laconic approach, with we, the audience, imbuing him with that underlying vein of psychosis because... well, it's Klaus, isn't it? He could lose it at any time! Crazy-by-casting, like you get with Dennis Hopper or Jack Nicholson.
Herzog got the real crazy out of Klaus anyway, in my humble opinion.