I'm divided between Kashimashi ~Girl Meets Girl~ and Sayanora Zetsubou Sensei. If only because neither of them are all that good for my mental health. But I still love them as series – they're just that awesome.
But, those are just top of the list. As for ones just below, that'd include: Ichigo Mashimaro, Ouran High School Host Club, and Nodame Cantabile. I like the manga versions of CLAMPs stuff better than the anime, so that's why I didn't include them, even though I adore Cardcaptor Sakura and Chobits.