I love Buttgereit. Nekromantik is one of my favourite low budget horrors. Further more, he's consistent - and Nekro 2, Schramm and Der Todesking are terrific films also (I bought the latter last time I was in Amsterdam). Art House horror at its absolute finest IMO.
Regards Olaf Ittenbach, I watched Premutos on a recommendation. It's fun, but a little too silly for my tastes. I prefer the arthouse sensibilites of someone like Buttgereit, than the hammy acted gore-fests of Ittenbach - although both have their place. It was quite a splat-a-thon, but again, with no substance I tend to find these a watch once, and then throw to the side type of thing. What's Nikos The Impaler like?
Andreas Schnaas, I regard the same. OTT, and rather cheesy splatter - but with a mean, nihilistic edge. It's like he has the same budget and gore capabilities - but throws as many taboos into the mix for reaction. Doesn't make the effects any better though, and offence for offence sake can be tedious. Problem is, i'm always curious enough to seek stuff like this out - then I kick myself for my disappointment afterwards.
I've not heard of Andreas Bethmann, so may check him out.. (recommended?), indeed my knowledge of German underground directors is next to none. I'd be glad to get some more recommendations.