There is no way GT5 will be the greatest games of all time. If GT5 leans too much to a sim there will be reviewers knock down it's score. If it leans too much toward arcade then other reviewers will knockk down the score. For example in one Forza 3 reviews took a hit in score because "it's was still too hardcore." They didn't like the fact around 4th season the races were getting too long.
To get the highest score you have to please pretty much everybody which may not be possible when it somes to sims /racing. I going to say GT5 probably get a point or two higher than Forza 3 if everything goes well.







