Bleach has a lot of good ones, but (as my avatar may hint at) I personally don't see how Cowboy Bebop could be any further than at least in the top three for anyone who has working aural pathways.
It's the only time I watched a show/anime I otherwise would have ignored whilst flipping through channels because of the introductory song.
Both the intro and ending are good, but I prefer the introduction (Tank!).
Yes, I know, I have a bias, but the reason I have this bias is the music, so I think I'm justified.