It depends, My favorite anime opening from this season is Kiznaiver (Lay your hands on me by Boom Boom Satellites). My favorite anime op of all time is The second Psychopass opening (Out of Control by Nothing's Carved in Stone)
I could be here an entire day. I might come back later. But of all time, as opposed to right now, I would say Diver from Naruto. Right now... hmm... probably Seven Deadly Sins from Nanatsu no Taizai.
fmab - again naruto - seishun kyousoukyoku soul eater - resonance hotd - highschool of the dead :icon_bigg
Blaze from Tsubasa Reservoir Chronicle, and Duvet from Serial Experiments Lain. Both set the mood for their shows really well.
Way too many to count, but for the sake of nostalgia, the first opening for Code Geass comes to mind (COLORS).
Dive in the sky which is the opening to Planetes or Eureka which is the 2nd opening to Space Brothers
Trigun (Vash is my favorite anime character) Cowboy Bebop Serial Experiments Lain Ghost In The Shell (both movies and the TV show... really the whole sound track) Tank Police (probably dating myself with this one) Gunsmith Cats