GT6 is a miracle of racing games. It is amazing what Polyphony was able to come up with, on such an old system and create something so fresh, fun and addictive.
I believe it beats Forza 5 hands down especially in terms of variety, driving experience and making you want to continue playing and learn every single car on every single track.
God, I'm probably going to regret this...
You're over-the-top enthusiasm really makes you sound like a Sony commercial. The differences are much closer.
Last gen? Sorry, but in the collective (which can vary immensely from your personal opinion, and that's alright) Forza 4 beat out GT6 on an old system!
GT6 is tied with Forza 5 so it's more subjective.
Lastly, Gran Turismo 5 beat out both of the new racers.