8+ is a kind of arbitrary line.
1-3 have next to no story and cookie cutter characters. 4 and 6 are widely considered masterpieces. 5 has a great battle system in the revamped job system, but it's largely forgettable otherwise, or so I'm told. 7 brought the series to the masses. I enjoyed it, but it has been a long time. I'd probably get a serious novela [as in latin soap opera] vibe.
I enjoyed 8, and I know I'm in the minority and I don't care. The complaints against Squall are only valid through about halfway. He shows the most growth of any protagonist this side of Cecil. Some will say it's too late in the game, but I don't care to argue the game. I enjoyed it.
I've played 9, but never got very far. The throwback mentality that drove it makes itself way too obvious and grates me a bit. I should finish it some time. I don't hate the game, but the franchise showed no real progress with 9.
10 has the best combat of the entire series, in my opinion. I enjoyed the story and most of the characters. Considering how many firsts are present in 10, I like it a lot. I'm playing X Remaster right now. I have not nor will I ever play 11 or 14. I just don't pay monthly fees to play my games.
12. I own it, have not played it more than an hour. I like the idea of the battle system, and it appears that the story is more politically driven, but I just don't know enough to give an opinion on it. I do want to get back to it.
I hated 13, and I hate Square Enix for absolutely ruining the game. Story was crap, characters were all crap, battles were crap, but it was pretty? The world they built sounds cool enough. The game sucks. And I hate saying it [the game sucking]; my wife bought it for me for my birthday because of all the hype I was exuding.
For me, looking at the series as a whole, their track record 8+ is pretty much spot on with what it's always been.
And as for the side games, X-2 and the 13 spinoffs, I'll play X-2 because I own it, but I'm not touching the other 13 games. I also feel like they don't really belong in the discussion. They're obviously games that Square doesn't treat with the same gravitas as the main line.
EDIT: I forgot Tactics and other games bearing the FF moniker. Some of them, like Tactics, are obviously great. But as above, the topic seems more concerned with the main series. So discussing those doesn't seem to add much to the conversation.