I'd have to go with Halo: Reach. Microsoft will put a massive marketing campaign behind it and treat it is a true Halo game, rather than a spin off like they did with ODST. GT5 I fully expect to be PS3's best game yet, but I think Reach will be the bigger success over the holidays. If this is going to be Xbox's biggest year yet, then Halo will be a big part of it.