I guess it depends on what aspect of the art you're focusing on. I've always felt that seminars come down to either specific focus, or hero worship.
If I want to improve my combative skills (CQC, EP, LEO) I'd always talk with Royce Gracie. If I was looking to improve my leg attacks then I'd go with Wolverine, Tonon, Lister. Take downs I'd go with Hector Lombard or Ronda Rousey. Guard passing I'd go Galvao, attacks Keenan or the Ribeiros. MMA I'd go with Renzo, Bravo, or a great MMA grappler, and my hero worship would be Rickson or Royler Gracie, Marcelo Garcia, Saulo Ribeiro, Roger Gracie, or other greats I've idolized.