Probably Lucifer Rising (pretty sure it was that, and not the one before. I'm not by my dvd's at the mo)

Its pretty much over-shadowed by the Sam/Dean angst angle, but when Cas helps Dean get to Sam, when he sends Zachariah away. I love it. It's subtle and shows he cares, more than he should, even after being re-brainwashed by Heaven. He went back to Dean. Sure it took a while, but the good things usually do :)

(side note, may I recommend the song Wish I Had An Angel by Nightwish. I think of Cas when I hear it.)