Not quite the oldest member
For me Forza 4 has the most enjoyable physics. I really enjoyed playing it, I remember playing F3 thinking although it was quite accomplished there was no feeling of fun yet as soon as I played the F4 demo I was sold, bought it day one.
For me, Forza 4 was an all around good Forza. Sure, it no longer provided LAN lobbies (system link), but it has a good set of on-screen in-race metrics and a high res camera, both of which disappeared from Forza 5 and 6.
Forza 6 (for me) is way inferior to GT6, and is why I bought a PS4 which is why I bought a PSVR and why I’ll get into “Dreams”, so it’s not all bad.