Converto.io

The best one. Let me list why:

1. It's not software you need to install, not shareware, not a "download helper", not bundled with other software, nor needs any permissions (logging into Google, Facebook, etc.)
2. The best features: always can grab MP4s and MP3s; can trim both MP4 and MP3; all MP3s have proper ID3 tags with album art; always offers the most video qualities (often 3 to 5+); allows changing the file name BEFORE you download (a godsend of a feature) so your downloads aren't cluttered with "videoplayback (6).mp4" or "video (3).mp4".
3. Ads are unobtrusive, always only on the sidebar, and no pop-ups.

Honestly, if you're using anything else, you're making your life harder.

I tried to download a video from YouTube via Converto.io, but it just gave me a still image from the video.

https://i.ytimg.com/vi/nncY-MA1Iu8/hqdefault.jpg