308 -- the one right after Brian tells Justin to have some balls. I read a fic once that said that kiss flayed Brian open (anyone remember which fic that would have been??), and it totally did -- you can see it in his face right afterwards. Love it.
I also like their first kiss in their reunion scene at the end of that ep, the tenderness of it and the way Brian waits for Justin to come to him.
Oh, and the one in the backroom at the very end of...311? 312? The one where they're devouring each other but we only get, like, two seconds of it?