IV and IX were tribute games, so those two need to be there.

II set the pace that every game would be different, you can't understand Final Fantasy without that.

V had a huge job system, it's also maybe the best balanced example of FF combat, add in good dungeons and it's a keeper.

VII, it put the series on the map and was the crossroads for the series, with some vestiges left over from the Nintendo games, but firmly in the style of the PlayStation games.