Definitely a tie with a few:

• Mushishi (The Sore Feet song and not Shiver, which is also great)

• Logos Naki World (Hellsing)

• Kibou no Kakera by Nana Kitade (Demsahita Powerpuff Girls Z op 1)

• Rewrite by Asian Kung-fu Generation (FMA '03 op 4)

• Detective Conan op 1 (the one from the Funimation English dub)