Like many others have said, I put my vote on SWG (I'm including Pre-Cu, Cu, & NGE because all 3 versions still had all those sandbox elements that made SWG so different from other MMOs; however, Pre-Cu had the most extensive features & realism) and even Eve being the most immersive MMO's I've ever played. Although I resent Eve for causing my Laptop's hard drive to fail and now the laptop is pretty much dead.
To really even say that, one must define what an MMO is or what the elements of an MMO are.