Awesome Belay and Rappel Biner
I have a Petzel William Screwgate locking biner as a dedicated belay and rappel biner. The Petzel William is very big and belays nicely with an ATC. It also rappels nicely with an ATC. I also like the added feature of the red, which visually lets you know if you are not locked. While there may be some marginally lighter options for belay/rappel lockers, I think the Petzel works great.