I started a thread on this, I would say butterfly XT or yyf whip, your choice. Both are responsive, cheap, and stay responsive for 3-4 + hours of straight play. You can add a touch of grease to the bearings so they stay responsive almost indefinitely, but too much makes it play more like a fixed axle.
The K2 isn’t bad, but in my experience after a hour or 2 of straight place it plays semi unresponsive because of the flush pads and the lube gets worked out of the bearing.