The is a simple userscript allows you to get raw youtube link without 3rdservice.
You could take a look of the source code, everything is fetch from youtube directly.
For FireFox users, go about:config and set dom.webcomponents.shadowdom.enabled to true to get the best experience.
What's the difference between Stream and Adaptive ?
Stream only support resolution lower than 720p.
So 1080p or higher needs to download from Adaptive. But adaptive media's video and sound is separate into two parts. To get full 1080p video, you have to merge them by yourself.
So how can I download 1080p video by myself?
Command line knowledge is required!
First, install ffmpeg. Then download 1 video and 1 music from Adaptive. I take mp4 and weba for example. Then open cmd/bash/... and execute ffmpeg -i INPUTVIDEO.mp4 -i INPUTMUSIC.weba -acodec copy -vcodec copy OUTPUTVIDEO.mp4