There are a lots of song that stuck to my head this anime season (R-V OP, SE 3rd ED, Kannagi ED....)
But for the past few days, the bloody song which haunt me every time i try to concentrate, even when i was on bed at bloody 3 AM last night, was "Paraside Lost" by Minori Chihara (or Garei: Zero OP).
I don't think it's as good as many songs stated. But the song fit the theme so nicely (IMO) that it refuse to leave and is killing my brain cells.