Probably FIFA with like 25 games or so. I only bought maybe 5 (my first was FIFA 1996) but I play it almost every year with some friends who own it. And not only the annual releases but also some World Cup and other games.
Second place is Halo with I think 12 Titles (all Halo titles, also counting Anniversary and The MCC)
Third should be the "Super Mario" franchise.
And then many with around 8 titles. Forza, Gran Turismo, NBA2k, Mortal Kombat, NfS and so on...







