Epic > Legendary or Fabled > Legendary
In Skyrim when during my first play-through I was around level 60 smith I was able to make a full set of Epic steel plate. I then restarted and now that my Smithing is around 80 I can make a set of Legendary steel plate but I can upgrade the thieves guild armor only to Epic.
I'm a bit confused on how things work in Skyrim, so I thought I'd ask which is the higher class of armor.