Where are you streaming to and what kind of content are you streaming?
If it's to YouTube, source is not available for viewers so VBR is acceptable, run VBR based on YouTube's recommended bitrates (5000-7500 for 720p) (8000-12000 for 1080p). Use CBR if you select the low latency option in YouTube livestream settings.
If it's for Twitch, set bitrate at CBR 3000-3500. I recommend 720p for twitch as you'll need a very efficient encoder to get 1080p to look good at 3000kbps.
In any case, use Quicksync to encode, use preset for maximum quality (quicksync should handle up to 1440p 60fps on max quality preset).
