So many good anime intro's and outro's out there. However, I only know really old anime titles so the music may not have aged well by today's standards. I really enjoy Megumi Hayashibara as both voice actress and singer. Watashi no Happy Birthday from Nuku-Nuku I enjoyed. Intro to Video Girl AI. Ending from Dragon Half. However, since I have to limit this to one intro I'll go with:
Give a Reason by Megumi Hayashibara from Slayers Next