For me it is FF XV.
I will start by saying all games till XV, despite problems here and there are not bad games by any mean. Realistically, FFXV as an idea must have been a 10/10 on creators mind, but a good idea is not materialised always in reality. Let start with the elephant in the room: the battle system
What the f... is this system. Clunky and feels glitchy when executing a move. The response is like 2 secs delay from button to reaction. The world, while open, it looks and feels like a bad copy of a big sandbox game, like Skyrim. And it is on purpose, but it doesnt feel as rich as any of the competition. This the first final fantasy where you cant control fully all players. It plays like a Kingdom Hearts in that manner.This is the first FF where the storyline has no meaning at all, based on how the character progresses and explores the world from start to end. Noctis feels exactly like he dresses, dead and black. If Lighting was perceived as not that real, Noctis is like a void on character development level.Now, go back to all FF, try to play them from the start till the end. You get directly the point that XV doesnt feel like a mainline FFs. While we loved FF7R for what it is, somehow FFXV cant redeem itself regardless of the DLCs, extra content, it just feels and plays dead and bad, as a package and these graphics cant justify it being a good game, cause the presentation lacks. On the final note, i think the series going multi-plat is what really caused this outcome: wanting to please Xbox playes pushed Square to a Skyrim type of game. Same thing happened with FFXIII, but to its core, it still felt like a traditional FF.