I use an online service, www.ssyoutube.com. You don't have to pay or install anything, although it often asks you to pay or download and install an extension, but you can simple skip it and use the free online version.
I personally recommend IDM (Internet Download Manager) I'm using it since then. It has lot's of features and for me it really suites my needs to download a video, music, files and etc.
for video downloading i am not using software but i use website to download youtube video to mp4 and save it to my pc . i use convert2mp3 (search it on google yourself . im not promoting/endorsing that website) despite the name it can download normal mp4 or if i need a fast download i can download the 3gp version . it is shitty quality but it is small sized file .
i used to download youtube with software like internet download manager but i just to lazy to install internet download manager and i used to use 4K downloader but im not using that anymore because it is doesnt work well for me and by the way nowdays i rarely download a video on youtube . i just watch it