Every time I upload a video to YouTube the quality looks horrible.

I'm happy this community thread exists.  Because of it, I've now started uploading my ProResHQ files directly to YouTube and there is a massive difference.  The issues I was having are with Adobe Media Encoder specifically.  I did not experience this with Apple Compressor, Davinci Resolve, or FFMPEG.  When the H.264 was taking scenes with higher ISO noise, or fast motion, even with a key frame for every frame 1 (GOP 1), it was interpretting the noise as less movement in the image, and essentially making the footage "blockier" and introducing more artifacts.  I'm assuming this because the H.264 export is optimized for speed and may or may not use hardware accelleration, but it was a real bummer to render out the same 4K24p footage in a dozen different ways, and have them all fail in certain scenes. 

I'm attaching an image: On Left is ProResHQ in s shot with motion, Left is Premiere H.264, then below is YouTube.  You can see that YouTube is adding another lay of degredation to the footage (making Premiere's look worse).  I can share the final video if anyone prefers, but when I upload the ProResHQ and let YouTube (takes a few hours) handle the re-encoding, I do not see the same artifacts as Premiere or AME.  It's actually been better quality to upload to YouTube, let YT convert it, then download the file from YouTube (although, I'm not sure where else you are uploading it to?).

I hope this helps someone else.