What's with all the Gran Turismo mentions?
GT5 is absolutely the best in the series, better visuals and by better I mean fucking fantastic, better physics (best on console sims imo), more cars and more tracks. Then you have the new additions such as kart racing, nascar, special events which are great, a course generator, day night cycle, weather, and a really good online mode. The series just gets better with each game, the only thing I didn't like was the constant loading and the load times, but patchs helped eradicate some of that. And the GT5 home page is much better than the home screen in GT4.