My all-time favorite album cover is
Relayer by Yes. In fact, I have the album framed and hanging in my room.
(You've already seen that image, so I won't re-post it.)
A close second would be Yes's
Tales From Topographic Oceans.
You've gotta love Marillion's
Script for a Jester's Tear.
After that, I think my next favorite would be Ritual's
Hemulic Voluntary Band.
Next, it would have to be The Tangent's [A Place in the Queue[/i].
After that, my next favorite is FROST*'s
Milliontown.
Finally, my favorite after all of those is Relayer's
The Teething Fashion.
I guess the last one isn't all that impressive, except for the fact that the kid looks a lot like I did when I was a little guy.
The only time I ever bought an album because of its art when when I bought Carptree's
Man Made Machine. The music did not disappoint.