Aria the Natural
I'm watching this again because I got the DVD sets for Christmas, but I could probably watch it a dozen times and not get tired of it. What Aria does really, really well is to immerse the view in a wonderfully realized and idealized world. No other anime I can think of does this so well. And that soundtrack [those acoustic guitars = love], easily one of the best. I like the second the season the most because it had a lot of episodes that dealt with the more supernatural side of the world [especially with Cait Sith].