Preferably one with more in-depth mechanics than just click to download. Stuff like you can choose a specific time-frame to download instead of the whole video or you can select the video quality. Bonus points if it also has a youtube-to-mp3 downloader.

On a side note, is there a way to watch youtube vids with shit internet without needing to download every single video? Maybe something like a way to force youtube to buffer the entire video like in the old days?