Knock Out is awesome! I also quite like:
+ Razorbeast
+ Team Bullet Train (CR/RiD)

I'd like to include Universe Overcharge if possible. I know he shares his name with the eHobby reissue of Diaclone Blitzwing... not sure if this counts. But even though Universe Overcharge is a repaint of a completely unremarkable Mini-Con, I really like the fact that he's a taxi! Such a rare occurrence for a Transformer alt mode!