Web/iOS/Android: Lots of web sites let you create playlist -- but most only use media from that site. Playmoss creates a master playlist that can include content from YouTube, Vimeo and SoundCloud.
While you can't import your other playlists, Playmoss lets you create new ones, supporting YouTube, Vimeo and SoundCloud simultaneously, so you can get all the songs you want in one place. Like most sites, you can follow other users' track selections on Playmoss, too.
Playmoss [Google Play Store]
Playmoss [iTunes App Store]