I like the Ouran HS Host Club opening, especially the part where the camera is spinning around one character at a time. (The best bit is at the end, when Takashi does this very cool-looking move where he flings his arm to the side and then back, as he leans forward - I don't know why I'm so impressed by that bit)
And though Ghost Hunt has the most boring opening to watch ever, the music is so good!
Gakuen Alice wins for most adorable opening I've ever seen, though. I love it when Mikan pokes her head out a tree, the world spins, and she lands on her head on the ground. =D