I would have said IX, but it's garnered quite a fanbase in the last decade. So V I guess.
Final Fantasy
Final Fantasy II
Final Fantasy III
Final Fantasy IV
Final Fantasy V
Final Fantasy VI
Final Fantasy VII
Final Fantasy VIII
Final Fantasy IX
Final Fantasy X
Final Fantasy XI
Final Fantasy FFXII
Final Fantasy FFXIII
Final Fantasy XIV
Final Fantasy XV
I would have said IX, but it's garnered quite a fanbase in the last decade. So V I guess.