Gravitation is by far one of the best Yaoi/Shounen-ai animes out there. In fact, it was the first I ever saw.
Shuichi is a dynamic character that makes you want to laugh and/or slap him half the time, Yuki is so cold that it's amusing to watch him and Shuichi interact.
Although it doesn't follow the manga exactly (ends well before the manga does), it is by far, still an amazing anime though it doesn't do it's manga counterpart justice.
It's a watch though. Watch it and be prepared to want to watch it again.
Not to mention the music is really catchy as well.
This one easily makes my top ten list.