I'm going to go with the original cover of Rifts by Keith Parkinson. it's just incredibly well-rendered bizarreness of a sort that really appeals to me. I still don't know what that thing (or things) is/are, and I don't really want to. Game stats would sully it, honestly. But Parkinson could really do no wrong.
Vault of the Drow (Erol Otis) is awesomely nostalgic for me. 1E DMG also gets the nostalgia vote. While these art pieces don't stand up to a nice piece by Brom, Keith Parkinson, or Todd Lockwood, they get props for shaping my young mind in weird ways. I've spent countless hours staring at them (and others from those days) and just imagining.
I'm awestruck by Komark's Spell Treasury cover and Targete's Arcana Evolved cover, but I don't want this to sound like an ad. Although while I'm on the subject, I honestly think the Ptolus cover is probably my favorite Todd Lockwood piece, right down to the embossed rat.
I'll also throw in Faces of Evil: Fiends from the old Planescape days by Rohb Ruppel. I must liked it, I bought the original. It's particularly cool because there's an omega symbol hidden in it, signfying that it was his last cover for TSR.
I'm sure there's stuff I'm forgetting. Like the first Dragonlance module cover by Clyde Caldwell, the chessboard series of Dragon Magazine covers, and tons more...