It's a tossup between my two favorite Final Fantasy games. The sheer mind-boggling size of Final Fantasy X-2 and the quality of the battle system, coupled with the cast of extremely endearing characters of whom I never tire and the promise of being able to gawk at Paine's sweet ass for the rest of my life, would definitely be a really great "just one game" sort of game, and not just because it's my favorite game of all time. A hot contender would be Final Fantasy XIV. I roleplay in that game, and the character I play is a really fun one. I don't think I could leave her behind, honestly. Yuna, as much as I love her, can get on without me. My Elezen pugilist can't, so unless online games are prohibited, my choice is Final Fantasy XIV.