I think Kingdra deserves it most, purely because there's next to nothing that can counter it effectively. It's one of those special Pokemon whose many polarized sets makes it near impossible to counter all of them. And while there are other Pokemon that can boast a lack of sure counters, for example that Darmantian by Pokematzer, they are often frail, requiring a free switch in to become effective. Kingdra has passible bulk and fantastic defensive typing, which means it's pretty easy to get in and start either spamming special moves, or Dragon/Rain Dancing to sweep. If it's a spamming Kingdra, switching walls in could be disastrous because a Draco Meteor could ruin them. But you'll want to switch in a wall to a sweeping Kingdra, before it gets a chance to start setting up multiple times because you switched in Draco Meteor fodder. Oh, and they can be from either side of the attacking spectrum, or even somewhere in between.
In short: versatility, typing, and the sheer threat it poses makes Kingdra the best Pokemon in UU.