Well it's not really a subscription but I use bookmarks for the most important channels but I think doing a subscription without an account is not possible could be wrong
News Explorer works nicely with YouTube RSS feeds, with enhanced full screen support and more. Disclaimer: I'm the developer of News Explorer, so this is not a fully objective answer
There are loads of RSS readers out there. Some good ones are:
* Feedly (free app, has both iOS app and website for e.g. laptop/desktop use)
* Newsify (paid app, has both iOS app and website for e.g. laptop/desktop use)
* Heartfeed(free or paid app without ads)
I'm only using those for news and not video so you should really try it out for yourself or check their websites/App Store description to see if they also support video RSS feeds but I think both Feedly and Newsify do that.
I do think the iCatcher! Podcast app (2.99 dollar app, once) together with the Podsync solution (convert a YouTube channel into an RSS feed) should also work.