What is the optimal bitrate for video quality?

This is a great question with no right answer. Instead it's important to consider what you're trying to accomplish and who is in your target audience. You might want to broadcast full HD but if the majority of your audience's internet connection cannot consistently watch at that level, then you're just wasting bandwidth.

For example, in order to enjoy a full HD experience at 1080p, you would need to broadcast at a bitrate between 4Mbps to 6Mbps. However when you broadcast at that bitrate level, less than 1% of viewers can stay consistently connected at that level of quality due to the internet speed of their device.

The Super Bowl streamed at 720p with a 3.5Mbps bitrate because they found that was the highest bitrate at which viewers could have a consistent playback experience.

We recommend the following bitrate settings based on this article from Teradek:

1080p  -> 4Mbps to 6Mbps
720p    -> 2Mbps to 4Mbps
480p    -> 1Mbps to 2Mbps
360p    -> 400Kbps to 1.5Mbps
240p    -> 300Kbps to 700Kbps