Well, if you talk about how well the anime was drawn in relation to the story, i would suggest "five centimeters per second." it has great art and a good sad plot that leaves you both happy and sad at the end.......
and i also like "Garden Of Words," "K","Clannad" and "Clannad-After Story." All of them show great art style in the drawing tagged along with a good. It really brings out the beauty of the story.
(note: the image is a screen shot from five centimeters per second)
5 Centimeters Per Second~ *pic
The most beautiful art style I've ever seen. I didn't like the story line that much but I just couldn't take my eyes off of that whole movie the style of it was just beautiful~
I also like Angel Beats art style! And lots of people bash Hetalia's first 4 seasons art style but, I thought it was really unique. It might not work for most other animes but Hetalia is just a light hearted funny crazy random show and for such a show that art style was perfect. Any other would be too serious for it~
For best art I'd definitely say Sword Art Online or Zetsuen no Tempest (spelling?). I also like Kuroshijitsu, Durarara!, and Death Note. Code Geass has amazing animations but the faces are kind of screwed up...
Hetalia. Despite the fact that they change the art styles, I still think both are amazing and fit the comedic anime. Here's a pic comparing the two styles for the character England/Britain(depending on the dub) The lower one is the older style.
These are mentioned a lot, but whatever. Two of my favorite art styles are easily Death Note and FullMetal Alchemist/Brotherhood. I also really liked the style in Attack on Titan! And Your Lie in April!