FFXV isn't the worst game I've ever played. It's not even my least favorite Final Fantasy. But it's probably the biggest lost opportunity of any game I've ever played by a wide margin. The combat is almost a lot of fun. The magic is really neat. The summons would be awesome if they were more controllable (some summons you will rarely see because of location criteria). The open world is cool, but mostly empty and completely removed from the EXTREMELY linear story.
But the worst offender is all of the story beats that are devoid of context or follow up. Apparently all of the backstory is spread out between the movie, a cartoon, and the DLC. But playing through blind it's just random stuff happening. The game had potential to be the most epic story if they could have stuck the execution of it.