It's a tough one to call. *I hate Cha-La Head-Cha-La*
There have been a few series that step it up as far as the OPs and EDs go. FMA and Bleach come to mind with a bunch of theirs. I also really liked the
OP to S.E. Lain (which has an awesome slide guitar riff in the full version) and the
ED of Chrono Crusade.
I know there are a bunch of others I'm leaving out. Some stuff for Inuyasha was very good and I personally liked the
ED to Midori no Hibi. The production makes the girls voice sound so soft and pleasing to listen to.
But the top marks have to go to the inevitable. Cowboy Bebop wins it for OP and especially for ED. What makes it so good is that the music isn't just good by OP/ED standards. It stands up to any music you can throw at it. There's no OP like Tank!, it has a solid and catchy bass line, and a great sax solo. And maybe I'm biased because I'm a bassist but The Real Folk Blues has one of the best bass performances I've ever heard. And it's played on a bass instead of a quantized keyboard bass-synth like so many OPs and EDs have.