I'm not a huge fan of Jeff Dee's art in general -- I find it a bit too "comic book-y" for my taste, but I find it really hard not to love this illustration of a mind flayer from Expedition to the Barrier Peaks.
This illustration exercised a huge influence over my young imagination. Having seen a mind flayer dressed up like an alien from an episode of Star Trek, I never again found it difficult to conceive of mixing science fiction in with my fantasy. Mind you, the entirety of module S3 was an object lesson in not being bothered by such things. Still, this piece of artwork has long been a favorite of mine. It's hard for me now to think of mind flayers as anything but aliens from another world/dimension.