I've played most of the CODs and Battlefields as well as Forza's and Project Gothams, but the only one i have gone back to again and again is Grid. Looking forward to Grid 2 next week.
Personally, I have had hours of fun with the Mass Effect Series. Skyrim gets an honourable mention, as does the Fallout series which I have enjoyed since before consoles, on the PC. Grand Theft Auto 4 and Fable 2 get a mention too :)
Couldn't pick a fave form the X-Box, fave of all time by a mile was Operation Flashpoint: Cold War Crisis on PC. Unfortunately, the crossover to X-Box and Codemasters going it alone with the game saw it sink faster than a whicker canoe, which was shameful.