We, the owners of EssentialMagic.com have collected over 45,000 artwork ratings over the years by Essential Magic users. Although 692 out of the 21,000+ cards we have in our database have all perfect scores of "5", here are the ones that received the most number of perfect scores, as of when this article was written:
1.
Sheep Token2.
Niv-Mizzet, the Firemind3.
Rakdos the Defiler4.
Korlash, Heir to Blackblade5.
Liliana Vess6.
Serra Avenger7.
Radha, Heir to Keld8.
Hellkite Overlord9.
Divinity of Pride10.
Richard Garfield, Ph.D.11.
Rafiq of the Many12.
Isao, Enlightened Bushi13.
Wild Nacatl14.
Twilight Shepherd15.
Orzhov Guildmage16.
Kitchen Finks17.
Hallowed Fountain18.
Dralnu, Lich Lord19.
Catacomb Dragon20.
Sorin Markov21.
Serra's Sanctum22.
Overgrown Tomb23.
Nightmare Lash24.
Necropotence25.
Myr BattlesphereDid you guess the Sheep Token? It's always the sheep.

...and where is Serra? #80...
Serra Angel
Here are some interesting articles about the best Magic the Gathering artwork ever done, according to the writer's unscientific (scoff, scoff) opinions.
http://io9.com/5976239/the-greatest-magic-the-gathering-art-of-all-timehttp://www.toplessrobot.com/2011/04/10_exceedingly_awesome_and_10_exceptionally_crappy.phpWhat are the worst? Keep reading...