Tried several, some online and some offline and concluded that one of the better ones is Free Youtube Downloader Version 18.104.22.168. It is very easy to use, fast and it doesnt seem to have problems with large files. It is not an extension of any of the browsers but a stand alone program. The interface is very simple and all you really need to do is to copy and paste the youtube video url in the space provided and press the download button. You can select the quality of the downloads -- I download my files as MP4 with the highest quality all the time without any problems. All files are saved in this default directory C:\Users\(user)\Documents\Downloads\Free YouTube Downloader, but this can be changed using the Settings option.
When installing the program just be alert and decline all the extras that the program wants to instal. Because it is free, it needs to get some revenue from these third party programs, but you can avoid these.
There are many youtube downloaders, and one or two with logos very similar to this one. To find the Free Youtube Downloader that I am referring to, use this link http://youtubedownloader.com/