To be honest, the k100 is a solid piece of kit. There are many other expensive mods out there with serial numbers and pretty logos, but that's about all the difference, really. People can argue about minuscule amounts of voltage drop, but mechanically speaking, performance doesn't get a whole lot better than a k100 with sanded contacts.
As far as the best, that's a very subjective question because what one person sees as a positive attribute, another may dislike. I am partial to Solid tubes. Some folks don't like them because they dont telescope.
The best advice I can give you is the same advice I give about ar15 lower receivers: find one with a roll mark that you like from a reputable company, and call it a day.