The TOS of Youtube doesn't allow downloads (unless they show a download link on the page). So that's why it's our policy to lock threads asking for software to circumvent their rules.
However, it seems that some of the tools a Google search find actually work. But we can't help you to select one.
I'd been facing a big problem these days because I'm working on Video Editing and sometimes I need to download YouTube Videos but there's no app which can let me do this. Please suggest me some apps.