Guitars are not precision instruments. It is impossible to perfectly tune a guitar. But in practicality, you don't need to perfectly tune your guitar -- you just need it perfect enough for the human ear.
So to help you choose the right headstock tuner, I've compiled information on some of the more popular ones, gathered from customers and personal experiences.
So of course there are dozens of other headstock tuners out there. For every one, there's two to three variations with little differences like color and material to display brightness and size. but these are the ones I see the most often and have the most experience with as a repairman. Obviously, Snark comes out on top and I think it's because their primary product focus is on tuners. They make great products and I stand by them 100%. Their interests aren't spread thin like some of the competition.
Personally, I use the Snark SN-5 for gross tunings for acoustics, but when it comes to intonation, it's absolutely necessary to use a proper stationary strobe tuner. There is no substitution.
Winner: Snark ST-2
Loser: Korg PC1 Pitchclick