Music wise; Shingetsutan Tsukihime - The Sacred Moon. As a whole, Tsukihime's whole opening (including animation) is my all time favourite, it had perfect execution to the awesome music. Other notable OPs IMO: Haibane Renmei - Free Bird; FLAG's OP; Freedom Project OVA - This Is Love; Bokurano - Uninstall; Gurren-Lagann - Sorairo Days.
Hmm.. EDs as well, eh.
For some reason I don't have really good EDs (eeeew Paranoid Android
). =/