While I do love me some 1970's pin art (Captain Fantastic & Fireball), I'm a sucker for 1980-era solid-states. Not only does the 80's have nostalgic value to me, but a lot of those art pieces really do stand the test of time. The 90's did produce some nice looking machines (Scared Stiff, Theatre of Magic, Cirquis Voltaire, and Monster Bash), but the sheer volume of them in the 80's outweighs them.
Fathom, Centaur, Firepower, High Speed, Earthshaker, Black Knight, Haunted House, or Eight Ball Deluxe. Lots of these have artwork so nice, that they usually end up on T-shirts (Haunted House t-shirts).