In terms of looks, I'd have to go with medieval or steampunkish fantasy. Games like FFIX, and Oblivion have amazingly well realized worlds. In terms of story, however, i would have to say Sci-Fi. Both settings have the capacity to tell any story they want (they are fantasy and can use majic/tech to get around the impossible), but swords and sorcery fantasy tends to repeat or at least borrow heavily from Tolkiens world. This is all a generalization I'm sure everyone can think of loads games that break the mould, it just seems to me that Sci-Fi settings have less of an obligation to conform to a pre-existing idea.