What a great topic! Some of my favourite covers are from sci-fi paperbacks from the 60s and 70s. If anyone is into cover art, then the book "Fantasy of the 20th Century" by Randy Broecker might interest you.
I really like when a publisher commissions similar art for a series of books, like in the Chandler covers posted above. Other favourites are the Tom Swift Jr. covers from the 50s and 60s, the "Lone Wolf and Cub" cover art, and the covers for the Turkish translations of Lem's works
http://english.lem.pl/gallery/category/32-iletisim
I can't figure out how to embed an image without a URL, so I'll just attach a couple of images of covers that I like.