^ Some story from last year (one that actually said the game would be named "The Old Republic") said that Bioware was looking at about 3500 BBY... a good amount of time after the series. All just rumors of course, kind of an old one, too.
Looking at games like Warhammer and WoW and even SWG, I'd think that the Mandalorian wars would probably be the best timeline, from a gameplay standpoint. And you wouldn't have to worry about Revan or Exile or any of the other series characters because their actions were all set in stone up to that point.
Based on what I've read around the net, the Mandalorian Wars seems to be fan favorite, too.