Do them all.
Actually doing them all isn't anything like as much as you'd think. They've been very sneaky with the trails and make it seem like they've got much more than they have by giving subsets of certain trails names of their own.
I think Twyr (or whatever the Red Bull is called these days) and The Beast used to cover pretty much everything of note.
You could add the Afon to that, but it's not great compared to the other routes.
The Minor Taur is additional, and is great for what it is.
But I think the MBR and Dragon's back are just subsets of the Beast/