Don't bother with Schloss Bensberg, which I found very overrated.

I actually had a very pleasant stay here four years ago: http://www.weingut-koegler.de/hotel.php?pSpr=en - it is not luxury per se but a very nice converted winery in a very pretty town which is well situated for exploring the German winelands.

Park Hyatt Hamburg is great. I've also had good stays at the R-C in Berlin and the Intercon in Cologne. The IC in Dusseldorf is up to the job although I'd be tempted to give the Capella a go.

Sylt is the super-luxury German seaside area - not got there yet but A-Rosa is opening a property in 2010 which should be good if their excellent family seaside property in Travemuende (search for my review) is anything to go by. The Grand Hotel in Heiligendamm is probably the most luxurious of the coastal hotels although this one is out of the way - this is where the G8 summit was held. It was a Kempinski until recently, not sure if it is better or worse under independent management.