I use
Youtube Downloader HD - Free Youtube Video Downloader.
I use it to get videos from Youtube to play on VLC on an old iPad 1 my 7 year old uses to keep himself amused during car journeys. Works well, provided I remember to tell YoutubeDownloader to create a 480p or less file.
It can produce FLV, MP4, AVI and AVI-HD files.
I use it to create MP4 files which it can do at 240p, 360p, 480p, 720p and 1080p.