The best settings are the ones that suit your needs and play style. That's the whole point of customization.

Myself, I run the G500 with a full load of 1.7g weights, with SetPoint settings of:
3 DPI levels 1300-1600-1900
Speed 0
Acceleration 1
Polling rate 333 reports/sec.

I do not use the OS drivers for pointer speed and acceleration, obviously (this is a checkbox in SetPoint).

My CS sensitivity is set to 3 (default). I don't have any checkboxes checked in the Steam mouse settings.

Belphegor: The G500 is capable of up to 5700 DPI, but the sensitivity at that point is so absurd there's really no point in being able to go that high.