One of my favorite bookcover designs is the original City of Saints and Madmen cover. While you can't read it in this tiny image, the entire cover is an actual short story, and the art is just incredible.
I always adored the space drawings you used to get on the Panther paperbacks in the 70's. Particularly Asimov stuff as that was what I dredged through second-hand stores to find. For example
I aslo really like the new edition covers for Neal Stephenson's Baroque Cycle, Cryptonomicon and Cobweb. Just can't wait for them to recover Snowcrash.
Similarly those covers for the Sci-fi masterworks series are great - and also meant also collectors out there had to make sure they bought them all.