Daredevil 189, and Daredevil 200. Yes, I read a lot of DD back in the day.
Various covers from Will Eisner’s The Spirit
http://www.dccomics.com/media/covers/2465_180x270.jpg
http://www.dccomics.com/media/covers/1197_400x600.jpg
http://www.universohq.com/quadrinhos/2005/galeria_eisner.cfm
Justice League of America #200
Crisis on Infinite Earths #12
I don’t have time to dig up URLs for these, but they’re pretty memorable. George Perez rules!
In my humble opinion, every cover of Planetary has been outstanding.
For something a little different:
Its Johnny Red! 
http://www.frothersunite.com/files/marbles/fanboy/imag/jred/cover_110879.jpg
Some of the drawing in that strip and others, in particularly Charley’s War, bordered on art.
Then of course, there’s Phantom Lady #17
The infamous Phantom Lady Bondage cover.
Frazettas covers are art…converted for cover use.
The Sandman #1 is the first one I thought of; I got it just because the cover looked neat, and it was the first comic I bought as an “adult” and got me back into comics in general. There are tons of other great Sandman covers, but Issue 25 is my other favorite.
All the covers to Cosmic Odyssey were just bad-ass. Also by Mike Mignola: Hellboy: Wake the Devil.
And Matt Wagner: Batman/Grendel. Nobody draws Batman better than him, IMO.
And second the mention of The Killing Joke.
