I don't really like the UFO Ultramaiden Valkyrie series. The art is attractive but the characters and story are stale to me. But one thing I appreciate about the series is that it gave me my most favorite tune of all, Natsu no Mukougawa, performed by Megumi Hinata, probably written by Kenji Kawaii.
I listen to it at home, in the car, at work, looped, and for hours on end. So even when I'm not listening to it, it's stuck in my head and playing constantly unless I finally engage in some other musical activity.