Sounds like you are running a sensorless motor? If so, then that's likely the root cause of your throttle response issue... consider investing in a sensored system, also carry back up sensor wires as they have a tendency to get damaged from time to time... when you start to notice any weirdness with the throttle, then replace the sensor wire... sometimes the sensor board in the motor can go out and very rarely does the sensor board on the ESC go out, but I see it happen from time to time.

I have tested close to a dozen brands of ESC's, simply put you will get what you pay for anything less than $100... by far the best quality budget brand I have grown to trust is Hobbywing... you can take some shortcuts and get their QuicRun 10BL120 which is based on the older V2 spec which lacks many newer features and requires programming with the older LED program card, but it will get the job done nicely. If money is no option, then I would consider the XR10 Pro Legacy which offers a very wide range of custom throttle tuning features to help get the best possible feel for your needs