I've only played Forza 2 and COD4: MW, but I think I gave both 5 stars if I remember rightly.
I thought Forza 2 was excellent and I actually liked it much more than Forza 3. I just liked the career layout a lot better, and I spent countless hours in the auction house - some people's designs were amazing! Can't compare it to Forza 4 as I'm yet to start that, but I do have it coming in the mail hopefully early next week.
Modern Warfare was one of the first FPS I played on the 360 and it's probably the game that got me really into the genre. Unfortunately Modern Warfare 2 didn't bring the same magic for me when I decided to play that straight after :( I still put COD4 in my top 10 retail Xbox360 games today though

I like what I like and I don't what I don't.