Internet Download Manager is good. I usually use video plugins from the extension store, but they get taken down by YouTube or don't work anymore.
I am also a fan of idm, between I do not download much videos, rather I will stream it in chrome, between keepvid was one of the youtube downloader I used.
I use a FireFox Extension -
Easy YoutubeDownloader Express. I've used it for a few years now and it's the only really good one for me that is free. If any one knows of a good free one for Google Chrome that would be awesome, but of course with Google if one has add ons you have to log in every time. I like the freedom of FireFox where I can have an extension without having to sign into an account.