Kk2 for me
Same. I tried a few other firmwares but it was when I was running some awful cheap props that made it unstable as hell. Now I've got a stack of Gemfans I need to retry them. Apparently the RC911 firmware is pretty decent, adding a hybrid auto level mode and making it far more stable than the stock KK firmware due to sensor tweaks.
Not that the stock firmware is bad, I've had no issues flying on default settings.