I have the cheapest Cherry Brown switch keyboard I could find, the Cougar Attack x3 Cherry MX Brown. I honestly have never tried any supposedly "good" mechanical keyboard so I have no point of reference, but this keyboard is decent enough for me to say that my lack of skill is the reason I lose, not my keyboard lmao.
That said, I think that laptop keyboards are also pretty good for GunZ. They're really low profile, making it easy to double tap your WASD keys. I learned the game on a laptop so when I got a PC I tried to look for alternatives. In my search I ended up buying 4 "low profile" membrane keyboards but they all have their own issues (either has something to do with rollover, or keys being too stiff, or keys being too high, or some sort of combination). I couldn't find an actual scissor switch membrane keyboard that felt like laptop keyboard, so I finally decided to give mechanical keyboards a try.
I'd still recommend mech keyboards (any switch that doesn't require heavy force to press) over anything else because I think they're better for the health of your fingers/hands. They usually don't require you to bottom your keys when pressing, so you can stop halfway after reaching the actuation point and lift your finger. That way you don't get the vibrating effect you get when you repeatedly tap something hard. Playing GunZ on a laptop keyboards feels like repeatedly and forcefully tapping a glass table and it gets unpleasant after a while. Membrane keyboards are relatively stiffer and require much more force.
IMO the best GunZ keyboard would at least be a mech keyboard whose keys have short actuation, has polling rate of 1000 hz, and has Full NKRO. With those bare minimum specs I think you'll be mostly unrestrained by your keyboard peripherals.
