A Lollipop or a Bullet is a really good series that IMPLIES shoujo-ai/yuri. I found it not only to be complex and interesting, unlike many yuri series I have seen. The characters are really well devolped and the art is beautiful. It's a series that's pretty realistic and could well happen to someone you know. I recommend this series over so many of the classics that are recommended.
*I said implied because, depending on how you view it, it could be a strong friendship or a romantic relationship.