Well, this is pretty hard to decide since I watched quite a few series so far and my favorites changed over time (e.g. I used to be a huge Sailor Moon fan and while I still like it, my enthusiasm has gone down quite a bit). But if I have to chose one, I'll guess I'll go with this:
Cowboy Bebop is simply one of the best anime shows ever created. Wonderful, layered characters, intriguing plots and a high re-watchability factor. The episodes cover all kinds of genres: action (many episodes), horror (Toys in the Attic, Pierrot Le Fou), romance (the Spike and Julia plot), comedy (Mushroom Samba), drama (Waltz for Venus) and everything inbetween. And let's not forget the absolutely bitching soundtrack by Yoko Kanno. See you, Space Cowboy... till the next entry, that is.