The best is the cover of Analog, by John Schoenherr.
The Magazine of Fantasy and Science Fiction had almost as good a cover, by Gray Morrow, and much better content:
The cover of Galaxy Science Fiction by [Douglas] Chafee is not bad either.
New Worlds and SF Impulse unwisely crowds out what looks like a rather nice image (by Keith Roberts, also author of Pavane and Gráinne) with text about the contents.
Finally, for completeness, Australia's Future Science Fiction has an cover by Keith Chatto: