I've been meaning to post this for a while now but I could never find the time. I hope I'm not too late, honestly I though this thread would last a lot longer than it did...

Soramimi no Cake (Azumanga Daioh!)
It always makes me smile and gets me in the perfect mood to watch the show ^_^

The World (Death Note)
The song speaks to me, it tells me of great ambition and the determination to make a change... despite that I can't say I like Light's methods for achieving this "bright and ideal world" of his -3- oh right, it also has some of the best visuals and music I've seen in any opening.

Days (Eureka 7)
This opening makes me feel free...

Rewrite (Fullmetal Alchemist)
I guess I like this simply because of the lyrics and music.

Keroro Platoon Authorized! Passionate Learn to Draw Song!!(Keroro Gunso)
Peole who don't know the characters might not get these but they always makes me laugh X3

Mabushikute (One Piece Movie 3)
Scenes from the crew's past TwT

There are others I'd like to add but I think I'll just leave it at that for now. Anybody else want to add before this thread is truly forgotten?