Oh wow, this is actually pretty hard for me to choose.
If I were to choose an OP that I felt fit the anime's theme best, I'd probably go with Shakugan no Shana's 3rd OP (Light my fire) or FullMetal Alchemists Brotherhood's OP Period (Forget which season it showed up. 4th or 3rd maybe?).
If I were to choose one I personally enjoyed, it'd be even more difficult. Probably Madoka Magica's OP or Shiki's. Maybe a tie between the two, even.