They've always used either Oster or Wahl where I've worked... but I can't remember which models. The cheaper ones you find at Wal-Mart or most pet shops won't be sufficient for extensive clipping on thick-coated dogs. My aunt purchased a $70 kit for me to buzz her Australian Shepherd with a few years ago, and the results were not pretty. (And pretty much killed the clippers!)
One of my cats hates being brushed and gets the occasional mat around his ruff area. To de-mat him, I actually bought a cheap-o pair of Conair human clippers for $9.99 at a discount store. I've had them for close to eight years now, and they are still going strong. (They may or may not have clipped a couple of humans over the course of their lifetime, too. 😳)
If you're set on something substantial, I'd check with a veterinary or grooming supply company... maybe ask a few local vets and/or groomers what they prefer?
(ETA- While cheap clippers may buzz through cat hair pretty effortlessly, I'm not sure how the results compare on a cosmetic level... I've never gone the lion cut route for Tyson. I just use them for quick, easy removal of the tiny mats he gets around the ruff when he's blowing his coat.)
