Probably Final Fantasy, because it has a lot of diversity and the overall quality is very high. I hate having to choose Final Fantasy because a lot of the games have a battle system that a large number of people are likely not to enjoy as much as is desirable. However, I can't think of any better choice either. It feels like I'm forgetting something, but I just can't think of anything else that would have a lot of variety and very high overall quality.
EDIT: Apparently someone mentioned Civilization, and I like that choice too. They're easy to learn but difficult to master, and there's something for almost everyone. You must really hate strategy games to not be able to like Civilization at least a bit. The games aren't that different from each other, but in a way they still play quite differently. I still think Final Fantasy is the better choice, but Civ is definitely an excellent choice too.
