Air, Byousoku 5 Centimeter Recommendations
Art, backdrop scenery, and color manipulation are spectacular in both. AIR fully develops the fragility of love and difficulty of human connection invoked in 5 cm. Both play on the theme of inevitable loss. 5 cm's brilliant music and use of silence is replicated in AIR.
Both have similar meanings in how the main characters can't be together, although they love each other. Both are amazing, sad and very touching.
Both series tell very sweet, very sad love stories. (Bittersweet, I suppose you could say.) They evoke sympathy for the characters without feeling at all contrived. There's a pretty good chance you'll at least have tears in your eyes by the end of both, as well.