For best quality, choose the highest upload bandwidth option and set H.265 to on if your hardware supports it.
In addition, add this line to your advanced configuration file (found at the bottom of your settings page).
That didn't fix the issue.
Parsec will automatically lower your quality if it senses your network connection has packet loss, high latency, or poor bandwidth.
Parsec prioritizes frame rate/latency over quality when there are network issues.