I use the Schampa high neck dickie
. I've used various neck gaiters and the famous buff, but none of them compare to this. Because it's a dickie and not just a neck gaiter, it does a far superior job of keeping the wind off of the neck. I usually wear it over my chin. This keeps me warm enough down until about 20 degrees F or so.
Below 20 F or so, I add a very thin balaclava that can be worn under the helmet. I also use a breath box (I actually start using that when it gets below about 40 F) to both cut down on the cold air blowing in my eyes and to avoid fog/frost on the inside of the visor. With the dickie, the thin balacalava, and the breath box, I've ridden down into the single digits at highway speeds. I wouldn't say I did it comfortably... But it works!