Strawberry JAM - Kibouhou from Spiral: Suiri no Kizuna is definitely my all-favorite opening. Probably because i really love that anime, such a shame when they didn't make anime of the rest of the manga.
and bottle fairy opening is just to cute to not love
For me it is "Lillium" from Elfen Lied.
Where else can you find such perfect balance betwen video and sound. Fotage features Gustav Klimt's artwork and for the soud is a marvelous opera voice of Kumiko Noma. Just magnificent, I'm on the seventh heaven every time I watch this opening.
it's appalling that gurren lagann wasn't posted until page 5, because that is by far the best, most fitting and epic OP of any anime ever, ever.
in fact, it was so great that gainax pretty much KNEW how great it was and didn't have to change the song for the entire series, unlike other shows that get bored with their own songs and go through 3 OPs in a season.
i'd post a video to it, but it's been pretty much eradicated from youtube.