This seems pretty compelling to me:

Once I got passed the fact that the 'action' is basically just a visual representation and started to play it almost as if it was turned based style I grew to love the combat. Trying to play it as hack n slash is pretty bad though.

FF VII has the worst combat. It's slower than molasses, bereft of any challenge for the most part, the majority of the magic is totally useless, the differences between characters is non existent and Materia is just a gimped version of Espers.

I haven't played XV so I can't comment on it, but out of the rest, I agree with everyone who says XII was the worst. Setting up a game to play itself is just terrible design; it was so boring to play. But then again, most of the series has had pretty bad combat, or bad character progression design, or both (looking at you VIII). The only real exceptions, IMO, are X and XIII; those had good combat, especially X. As far as MMOs go, XIV is really solid too, and Tactics is a spinoff but its still the best isometric, tactical, turn-based combat I've ever played (just a shame how easy it is to break it).

So if XV does have bad combat, that's just continuing a long tradition.

At first i thought the combat was really simplistic but there's a hidden layer in there ala Dark Souls that makes me keep finding new stuff.

I haven't played 15. 13 is perhaps one of my most hated games of all time that I actually finished. I have never been to a point where I was so angry at a game. The combat eventually gets ok at points in FF13 but man I just remember spending 95% of the game making no interesting choices just spamming the x button [not x to make menu choices, just to do the auto battle or whatever it was. Maybe I have the button wrong that is just what I remember but you get the point] and walking forward. There was just no reason to do next to anything else in that game other than a few rare occasions. I honestly am just not willing to give another FF game a serious chance after what I consider the completely mindless combat system of 13. Yet anyway, maybe eventually if I hear something really does get special.

Then again, other than XI the pre 7 games are the only ones I actually enjoyed with V being by far my favorite.

Despite some camera issues I quite like FF15's combat; I find it fast, fluid and fun. I can see how if you were expecting/wanted turn based tactics or traditional ATB then it would be disappointing.

Even though it's one of my favourite games, I think FF9 might have the worst combat in the series. Too slow and the skill mechanics are ridiculously simplistic.

I'm working on a video series explaining how the combat works in this game; I'm finding that a lot of people don't understand how it works (not in a "I hate it, it sucks" way, they actually don't know a lot about it), and that might be coloring people's opinion on the game. @fredchuckdave posted a good video explaining why the combat is--at the very least, not the worst. There's an expression of skill that a lot of Final Fantasy games can't afford in their systems. The demo video I recorded to show off the topics in practice was a level mid-20s fight against a level 52 boss called Aramusha who has Instant Death on a lot of attacks and it's pretty cool that I was able to kill that guy at all.

7 and 9 are pretty lame. 7 gets going in the end game but before that it has a baby JRPG combat system for babies.

13 is my most hated. There is a ton I could gripe about, but the bottom line was that it just wasn't fun for me. I bounced hard off of 12 when it came out, so I don't really even remember enough to say one way or the other. I never fully grasped 8's combat,but for me it was kinda annoying st times. Other than that, I liked them all. Haven't played one before 6.

The series is too big and too different to have a "worst" or "best". You can see that by everyone having wildly different views on it just in this small thread. This question is the equivalent of the "which to play first" or "best FF" that pops up. However...

I think XV is good but the fact is they never got that camera wrangled all the way and it shows. I hope I get better at the combat in general though. I personally really like VI because of how different all the character mechanics are, and doing fighting game motions with Sabin was awesome. I also actually enjoyed XII a lot because it felt like logic gates and coding. I don't remember taking issue with IV, V, VII, VIII, X; they were all fine but not memorable for me as combat goes. I did enjoy XIII a lot at times for the overall "group" type combat, but your individual picks mattered very little and I hated when they brought the group size down to 2 all the time.