Grandmaster first appeared in Avengers #69. Grandmaster is a powerful being who is obsessed with games, particularly ones involving one side against another. He has used many Marvel (and even DC) heroes and villains as pawns for his games. Despite this, however, the Grandmaster is not truly evil as he even follows his own rules and even gave up his own life do to a promise he made to the heroes of Earth.
French Tickner: French gives Grandmaster an older yet wise voice and the echo added on gives him an otherwordly feel. It's not bad but I feel for a character of this nature needs someone with a bit 'oomf' in the voice, something that gives him more of a presence. French just seems too light and stiff in some lines. He's still good but I think he could have been better.
John O'Hurley: John plays Grandmaster as more of a host/announcer which John has experience with. While John doesn't do much to differ this from some of his other voice work I think John has that "oomf" i thought was missing from French's take.
Jeff Bennett: Jeff actually reminds me of French, he has an older yet powerful feel to him. Jeff also does well with making Grandmaster sound obsessed with his 'game' and makes him threatening but not too evil sounding.
Jason Spisak: Jason's take is different than the others (input "why is Guardian's version than Ultimate Spiderman's, bla bla bla bla), his take is more flamboyant and focuses more on the "showman" side of him but still has a sinister edge to him. I should consider him the weakest but I find Jason oddly entertaining and I can't really call him bad.
All four did a good job but I think I'm going with Jeff more.