Well,
Abbey Road, of course.
And, just as obvious,
Dark Side of the Moon.
Californication has a really haunting photo on its front cover.
Most of Peter Gabriel's albums have really compelling cover art, but especially his self-titled ones, and
especially the third one:
I've been doing a lot of research on albums that were important to me in my youth, as part of a column I put up weekly on Google+ called Nostalgia Monday. I'm finding the album art on a lot of these albums to be more engaging than I remembered to gave credit for. In particular System of a Down's
Toxicity is a perfect embodiment of the theme of American corruption that pervades the album:
Here are some others that have struck me recently:

"Warren Zevon would be proud." -Reve Mosquito
Stages, an album of about dealing with loss, anxiety, and grieving a difficult year, now available on Bandcamp and all streaming platforms!
https://jonporobil.bandcamp.com/album/stages