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. 

i only played... ff8, 9, 10, 12, 15

ff8 for me... dont like the junction system...

ff12 grid system was my fave

OP, I think you should check the original Final Fantasy I on the NES. That thing is so bad it gave me nightmares. WTF is wrong with the accuracy system? Half the difficulty of the game is fighting against the RNG to actually land a frickin blow. And what's up with the speed stat? The game seemed to just randomly decide when everyone attacks. I guess that's where the other half of the game's challenge comes from. Sometimes my party attacks first, sometimes I have to wait for the entire group of enemies to destroy me. Which happened to be the exact same enemies that attacked after me the last time.

I guess the game was good in 87, but today is garbage. FF XIII and XV are mediocre games, but at the very least you can play them without wanting to kill someone...

II probably. It had a lot of interesting ideas regarding character leveling, but none of it really worked in execution. Of the modern ones it's XIII most likely. I think in a similar vein XIII had some interesting ideas, but it failed to really do anything with the potential it had.

Final Fantasy 2. I can imagine most people dont take it into account because they havent played it. That game is a trainwreck in every aspect.
Final Fantasy 13 would be my second-worst pick, but between 2 and 13 theres a world of difference. 13 is mediocre but at least has some enjoyable aspects and good values to it.

I'd say its either gotta be FF13 or FF15 honarable mention being FF8.

FFXIII Franchise. Each one of them is a soulless pile of shit. Convoluted nonsense story. No characters in them just caricature at best. Hallway simulator. Side quests in LR were meaningless. I somehow felt claustrophobic in XIII-2. You have to make your brain numb or have to be numb already to enjoy this vapid wasteland of games.

FFXV is bad but I was still able to kill some time with it and get a Plat on PS4. Not sure I will revisit it. Unfinished mess. Playable mess. At best this game has a small select few highlights of spectacle. That's the main issue with modern FF games. They are more spectacle than substance.

Final Fantasy II is the worst of the classic era.

There has not been a single good entry in mainline FF since XII. The Series has not been truly great since IX.

why so many saying 8? FF8 is an enjoyable game, sure it takes a bit of getting used to but whatt FF doesn't? for worst i nominate 5.

It's not a bad game but a bad Final Fantasy and that is where the confusion lies I think. FF is a franchise with grand epic adventures of saving the world. With spectacle. You look at the depth of VI and VII. Lots of character. FFVIII sadly had to follow FFVII and it was such a step-down and Squall was such a grump. The story wasn't as grand and then there is the Junction system. Xenogears began as Final Fantasy VII and probably could have been FFVII if they wanted. If they let them finish Disc 2 then it might be one of the best if not the best FF game ever made. I see FFVIII in the same boat as Chrono Cross. People rag on CC a lot. It's a good game but a bad follow up to CT. If they were just their own game as a new IP I think they would be more highly regarded.  Similar to Vagrant Story. I don't think VS would be as loved if it was a mainline FF game. Just too different.

II. There are other sub-par entries in the series, but honestly II is barely even playable, especially the original NES version. It's a shame too, because it has a much better story than I or III, but its levelling and battle mechanics are so ridiculously ill-conceived that they just kill the whole thing.

XIII would be a close runner-up, as it's just far too monotonous. Sorry, but if I have to wait a length of time the equivalent of an entire playthrough of any of the first three Mass Effect games until it "gets good", it just isn't worth the effort to get into it.

(I suspect I wouldn't have thought very highly of XV had I played it on release, but I bought it a year later with all the DLC, and actually thought it was pretty decent. If nothing else, I had more fun in the first hour of playing XV than in all the time I slogged through XIII)