I'm going to take a slightly different tack and go with my favorite albums of or in the style of the first couple of generations of death metal (I would not consider The Red in the Sky is Ours or Nespithe or, well, any of the albums listed by the first couple of posters to be 'old school' simply because those albums are far too advanced when viewed in relation to the genre's early history).
Anything up to 1995 is old school in that Death metal was continually being tweaked and shaped, but it creative eventually peaked as surge of new bands stopped and the older bands continued to push new material. However, I do enjoy your approach and believe it shall be very helpfull to many people on here. I'd add Dream Death to that list