My favorite yaoi series of all time is Haru wo Daiteita by Youka Nitta. It's...hardcore yaoi, so probably not everyone would like it, but the love between the two main characters is so utterly SWEET. ^^;; And Youka Nitta's one of the leading yaoi manga-ka in Japan. I also love Gravitation, and Clamp shounen-ai, and am told I should really watch Loveless. o_o;;
I just finished watching Gakuen Heaven, and I loved the series. I watched it almost nonstop for 2 days, and was disappointed that it ended at only 13 episodes. (Why is it that most BL series are short?) I heard the manga is way different though.