In Unova, Terrakion was my biyatch catch. Virizion was a very simple one by comparison. Guess it differs for everyone. Landorus was also difficult as I forgot my paralysis user.
The Shiny versions are Events, and I had to generate my own copies of them. They are not yet imported. Far as I knew we were only trading Registeel, but I have clones of the three beasts as caught in SS in my Unova PC.