Mönchshof is delicious, especially when you can get it relatively fresh and on tap. I also like Krusovice. It is less complex than Mönchshof, but it is usually available for a pretty reasonable $2.50/500 mL bottle.
My favorite used to be Kreutzberg, which was rated the best beer in the world by Playboy about 25 years ago. Now, it has been too many years since I've been to the monastery and I'm not sure they make their Schwarzbier any longer.