I have a varied taste, but my favorites would have to be (in no particular order):
Blue Gender
Gundam Wing
Queens Blade
Revolutionary Girl Utena
Umineko no Naku Koro Ni
Honorable mentions: Ah! My Goddess, Armitage III: Dual Matrix, Cardcaptor Sakura, Cowboy Beebop, Gurren Lagann, InuYasha, Noein, Outlaw Star, Ninja Scroll, Sailor Moon, The Melancholy of Haruhi Suzumiya, Trinity Blood, Ronin Warriors, Rurouni Kenshin, Sailor Moon, Sakura Wars, Simoun, Tenchi Muyo, Tokyo Japan, Witch Hunter Robin, Wolf's Rain, X (1999), Yu-Gi-Oh! (Second season to pre-GX), Yu-Yu Hakusho
I'd also like to potentially add Sword Art Online, it is getting to be a favorite of mine (haven't watched past Ep.9 yet I think it was).