Unless you started climbing only within the last several years, chances are you've owned a Black Diamond ATC belay device. This is the archetypical manual belay device. Even with all the new devices on the market, this is still a great one. The ATC is especially smooth when belaying 10mm or thicker ropes. There's no need to have the friction grooves found on the ATC XP when belaying with a rope that already provides ample friction. The ATC works on thinner ropes, too, such as a modern 9.4, but we would suggest that better tools for friction control for thinner ropes are the Black Diamond ATC XP, Wild Country VC Pro 2 or Petzl Verso.
The Black Diamond ATC is simple, light, affordable, provides smooth rope handling and is compact. The ATC design hasnt changed in 20 years and is competitively priced. It doesnt have friction grooves, heat dissipaters or any other fancy add-ons it is simple and works great. If you rappel a lot, get something more durable such as the Wild Country VC Pro 2 (it has thicker tube stock and friction grooves). The ATC works exceptionally well on ropes 10mm or thicker. If you mainly use thinner ropes, a device with friction grooves such as the Petzl Verso, Black Diamond ATC XP or the VC Pro 2 is more appropriate.