For me, two openings comes to mind, and i'ml mostly judging them on the music.
The soultaker, the first anime i've watched while knowing about the existence of anime (it mean : not Sakura Card Captor, i was too young). The anime is barely watchable, it hurts your eyes, the story is pure shit, and so on. It is just boring. I've try to rewatch 4 times, i've abandonned at the 6-8 episode each time !
The first OAV of Agent Aika. It's nice to watch once, it even have some charm, but let's face it : it's a bad ecchi anime.