Not really a tough decision.
This show is god damn pretty. And pretty relaxing. It’s got some of the most interesting designs I’ve ever seen in anime, and some of the best animation I’ve seen. It’s such a wonderfully Eastern work, steeped deeply in Japanese mythology and the philosophies/morals presented in its folklore. It feels a lot like I’m watching a work of art, and its art reflects that.
That being said, watch this right now. It’s one I will always recommend. It’s relaxing, though, so don’t try to marathon it. Just watch one to three episodes per night, and if that doesn’t relax you, then overdose on the series. If that still doesn’t relax you, you’re eternally tense and I can’t help you.
It’s something you need to experience for yourself, which is why I didn’t drabble on and on at length about it.
Runner-ups: Kino’s Journey, Hidamari Sketch, Puella Magi Madoka Magica, Serial Experiments Lain, Haibane Renmei