I think CB is the best because it has all the things I love all in a single anime. And it is the show that got a lot of people into anime. Faye is my personal hero and role model btw lol.
I can see it at no. 1, but it just depends on if you like the sci-fi genre or not and the type of rambling, round-about story. I've heard people criticize it for having too many filler episodes. It's a classic to me, but probably not unchallengable.
Cowboy Bebop is the first anime I ever saw. I think it has a special place in anime history. I have nto seen enuff to say if it is the best but I think it is a strong candidate.