This doesn't make sense to me. Are you comparing 1997's GT with 2008's GRID? Obviously GRID would be better in almost every conceivable way. I guess I'm supposed to factor in the date of release, in that case:
Gran Turismo > GRID > Forza
Now, it may be said that any Sony fan would say that, but if we talk about the latest instalments then I have to mention that Forza beats GT in my book. Driveatars are very cool, and there are other things that make forza better imo.