I said the SS Vader Exclusive, mainly because of the size and the real materials.
The Attakus Vader I is the best sculpted version in terms of the helmet, and has nice paint apps.
If Cinemaquette ever makes a Vader, then I'll be selling the SS one for that. The SS one is not accurate, but there is no scaled Vader out right now that I'd rather have.
GG and the Koto one are just bad, IMO. They are decent values if you don't want to spring for the SS or Attakus Vader, though. I've seen GG Vaders go for less than $100 on Ebay, so that's a good price.