It's hard to determine who is best written because different people enjoy different writting styles and different novels.
I, for example, love Luceno, Stover and Stewart. So Darth Plagueis, Revenge of the Sith, New Jedi Order: Traitor and Yoda: Dark Rendezvous remain my favourite novels of the EU.
But someone else might like Drew, Vonda, or Christie Golden so they'd enjoy masterpieces such as Revan, The Crystal Star and Dark Disciple.
It's simply a subjective opinion on what writting style you enjoy, and how much you idenitfy with the character and their story if presented through the writting style.