Which ones, either currently or from the time that they were made, really pushed the bar for animation? This includes character designs, style, and movement/fluidity.
Feel free to post vids, and argue and whanot :)
Now, while I haven't seen a whole lot of animes, I must say that Casshern Sins, and Hellsing (during the fight scenes, at least) are great looking. Mostly, I think, it's how fluidly everything moves, although I really like the general character designs in Casshern.