ITT we discuss anime with superior art style. This doesn't mean "oh wow the anatomy is all correct, I'm glad they did it RIGHT", but rather stuff that is amazingly original and beautiful and feels like unplanned-for electric shocks in the cerebral cortex.
My favourite anime of this type has to be "Kyougoku Natsuhiko - Kousetsu Hyaku Monogatari" (aka Hundred Stories). It is full of absolutely stunning use of colour and form. It shows enormous self-confidence by using large single-coloured surfaces of pure white, fog that looks like pencil eraser marks, pitch-black shadows and mixing all this with intentionally-bad CG animation - and it gets away with it.
When you watch this anime, nothing matters anymore: you know from the first instant that The Deed is Done, you want it or not. A million tiny needles of art pierce your mind as you get elevated to the Next Level.
Azumanga Daioh
Cowboy Bebop is visually stunning, with gorgeous CGI; it also has stark bright colors, pitch darkness, and drastic contrasts that evoke film noir. The character design is also mainly straight out of 1940 by way of 1971.
Neon Genesis Evangelion and End of Evangelion are likewise beautifully animated (especially note all the reflections on the lenses of Gendo's eyeglasses, and remember that NGE was made in 1995--years before the tools to do that as CGI were common and cost-effective), and the giant mecha fight scenes are, particularly in EoE, absolutely breathtaking.
Wolf's Rain is also visually attractive, with dull, drab, washed-out colors that evoke a sense that the world is dying.
Too bad Wolf's Rain didn't have anything ELSE going for it. Well, those aerial battles were incredible, but there was something like three minutes of that in the whole series.
Purely visually, few things beat FLCL for me. There are many, many things I like about FLCL, but the animation and art is just mind-blowing. The style mixes japanese and western elements wonderfully (aside: Jamie Hewlett influence?). And the animation itself contrasts radically different types of motion for maximum impact - slow motion is contrasted with incredibly rapid movement, gravefully floating motion with hard impacts, and simplistic art with detailed.
Porco Rosso did (and still does) it to me. Just the sheer beauty and craftsmanship that went into its artwork (like a lot of Ghilbli filmes), from the reflections, shades and the way water/sea was drawn. It has none of the off-moments I find in most very recent anime which has badly drawn lines, or innaccurate bits as a result of two different artists/studios doing the same ep/series. Clouds and foliage is soft in visual texture, whilst the edges of aircraft are sharp and consistant. Maybe my liking for it is fueled more by the fact that very little, if not any CGI was applied to the image, given how old the movie is.