I tend to get Getsurenka, Logos Naki World, and Kataritsugu Koto stuck in my head very very often. Not that I mind, they are all good songs.
Getsurenka is from Shadow Hearts: Covenant (this is one of the most touching, heartbreaking endings to a videogame I've ever heard.)
Logos Naki World is from Hellsing,(it's the opening!) and
Kataritsugu Koto is from Blood+ (it's the first ending theme.)