Raspberry pi? Usb asp is probably the cheapest and easiest if you want to call it that. It really wasn't too hard to flash but can be a little time consuming as things don't work the way you want them too. (Computer not up to date, making a wire harness to plug onto the kk2, choosing the file to install)
It should fly ok on the stock firmware if the PID is tuned in. It make a noticeable difference when I reflashed mine. I don't think I even messed with the PIDs after I flashed it.
