Which company would make "better" exclusives is quite debatable, and very subjective based on a person's preferences for what they want in a game. Which company would sell more might not be possible to call; Microsoft probably has the best advertising, but if any of the 3 came up with "the next big thing" (for lack of a better term) they could take the lead regardless of the size of their ad budget.
I think another interesting question would be which companies would be robust enough internally to maintain a platform with just new IP, based not only on making games that appeal to people, but also crank them out often enough to maintain a deep lineup of games so people don't go for long droughts without new games. From this angle, Sony and Microsoft should both be fine. Both of them make quality games that plenty of people enjoy; Sony has the edge in terms of the number of studios they own, while Microsoft has had the edge in terms of actual sales (and they have been beefing up their 1st party side lately, recruiting talent for Black Tusk and creating Osaka, just to give a couple examples).
Nintendo, on the other hand, might be in a bit of trouble. They are well known for working hard on their games and coming out with quality products, but they rarely create new IP and they often see delays on their games. They could strike it big with another Wii Sports-type casual hit, or a more traditional Mario-type hit, but if they strike out with a new IP that can't leave niche status (see Pikmin) they could be in a world of hurt if they can't crank out another game quickly enough.
