For me, the 135mm f2L. I find it crazy how long this lens has been produced, and have loved it ever since I finally got my hands on one. Doesn't take up much room but can do things few lenses can do. One highlight was taking headshots in an office using a single strobe (with HSS) and shooting at F2. Watching the backgrounds just blur away still gives me a little rush, it just delivers consistently every time.