Depends on how familiar you are with the FF setting and the FF world, and especially the FF7 setting. If you haven't played FF games before, then it's easier to start with FF16 - it's a simple linear game that goes through pretty quickly. But I'll say right away that FF7-FF7R are objectively better games, but you'll have to buy two at once since they are connected by a plot, and if these are your first games in the FF7 universe, then it's better to start with Crisis Core: Final Fantasy VII Reunion - since it's actually a prequel to FF7 and tells about how it all actually started and where the game's legs grow from. Because FF7 actually starts from the moment Crisis Core ends and the game kind of assumes that you know the events that happened there.
Although to be honest all three games are passable, and you are unlikely to ever replay them, the only difference is that in the case of FF16 you just need to buy one game and play through it, in the case of FF7, if you want to understand the world and the history of this world, you will actually have to buy 3 games.