I use Firefox with an Add-on called “Flash Video Downloader”
1. install Firefox.https://www.mozilla.org/en-US/firefox/new/
2. Then Click on the Menu iCon and choose Add-ons. Choose Extensions. Search for “Flash Video Downloader”
Install Flash Video Downloader
Now you will have a “Download” Menu under your YouTube Video. The top video shown is usually the best one.
Save the Mp4