I have used many different tire deflators over the years.
An inexpensive option that always works, has no moving parts, and costs very little is this one from Teraflex, usually about $15. I have carried one for more than 25 years, now as my back-up:
Claimed to be the fastest are the ARB and Currie units that remove the valve core, but you still have to bend over and fiddle with the pressure on each tire, $38-40. [I do not recommend the S*ittybilt and other cheap Chinese knockoffs - sure they cost less but you get what you pay for.]
The most convenient are the automatic tire deflators such as Staun and the improved version sold by Harry Llewellyn at Coyote Enterprises - $56-72 depending upon the model and where you buy them, less for the Chinese knockoffs. Harry was the face for Staun in the U.S. until the Australian company went broke in 2009 and Harry still has a working relationship with the original patent holder in Australia. However, his Coyote Automatic Deflators have several improvements evidenced by a new patent held by Harry and are a step up from the Staun and Staun II deflators. Coyote Automatic Deflators are far superior to the cheap Chinese knockoffs such as the EZdeflators referenced in Post #3 and the deflators sold with the S*ittybilt brand. [I have a set of the EZdeflators and find them difficult to set. They also produce inconsistent results which makes them no bargain.)
Shown below are my Coyote Automatic Deflators. They came pre-set at 12 psi but you can have them set to any pressure you desire and they are easy to re-calibrate when you want to change pressures. The advertised range is 4-56 psi.
Unlike the knockoffs, Coyote Automatic Deflators always deflate to exactly the same pressure, never stick open or bleed air, and you can put them on and drive while your tires deflate unlike the other brands. I can air down all four of my 35" tires from 28 psi street pressure to 12 psi trail pressure in about 4 minutes which compares favorably just over one minute per tire with the ARB deflator and I don't have to bend over to manually set the final pressure as with the ARB product. I think they are a good buy at $56 ($60 if Harry sets your custom pressure) and well worth the price difference over the imitations.