I don't think I can choose a single "best animation". Too much good stuff to choose.
I like some of the "big guys"---David Firth is amazing and a true artist...David Lovelace of Retarded Animal Babies fame caters to my immature side...Krinkels re-invented the mindless violence genre...etc.
I think Adam Phillips is damn good with graphics, although his movies are kinda low on plot. I dislike illwillpress---he's an overrated hack.
I also like some less-known artists. ZekeySpaceyLizard is well-known to Newgrounders (although, sadly, not as well-known as fucks like illwillpress), and his work ranges from spammy to silly to epic. Mottis of Bunnykill fame is perhaps the only artist to successfully continue to Madness genre. LeonardRawles is producing an amazing series called Demithyst that blends several animation styles into a masterpiece.
And I like some of the so-called spam artists, as well. AfroUnderscoreStud and me collaborated on a number of admittedly shitty flashes, but sometimes humor is more important than graphics. On that note, the late Barney Bunch was a huge source of entertainment; I don't think the Kitty Krew has picked up their mantle.
Good stuff.
The-Red-Jack
What is Love?