Does it have to be TF2? 'cause my favorite kill ever is and will likely always be a GTA2 kill. In TF2 it all kind of blends together for me because our games are so massive and you rarely have time to stop and enjoy your accomplishments. But, if I had to venture a guess in TF2, I had a great one the other day.
I forget who it was, a regular in the PL maps server. I was engi, he was an enemy spy, so naturally we were butting heads. Every time he'd sap my things, I'd get all the sappers off, but he'd get away. My tele exit was a bit further away from the gun/dispenser, so he just sapped that to bait me. I walked into the room that was in, and saw him there, disguised, ready to jump me. I went down to the exit, hit it once, turned around and saw him jumping over me to stairstab me (tele was at the bottom of a small stair flight). I followed him over my head, and with the next swing of the wrench, crit him in midair and took him out. Swung it again to finish off his sapper.
Unfortunately my team lost shortly thereafter and he glory round killed me.