There's a great choice of formats – both video and audio – including MP4 and MP3. To grab a video (or a whole playlist, provided it's not longer than 24 videos), simply copy its URL from your web browser, click 'Paste URL' and select an output format, quality and location.
It's simple to use, highly customizable, ad-free and doesn't include any bundled extra software.
4K Video Downloader is the best free YouTube downloader around for quick, fuss-free YouTube downloads.