It's always gonna be subjective, but most people consider FF6 to FF9 the "golden age" of Final Fantasy. Each one offered new, groundbreaking ideas for the series if not the entire genre, and each were immensely popular at the time - with the exception of 9, which was released so close to the end of that console's life (and FFX's launch) that it went mostly under the radar.
Hipsters like hating on 7 because it scores them wicked internet points, but it's typically regarded as the most solid all-around. The graphics (for the time) were outstanding, the story surrounding cloud and the build-up to the final confrontation is literally unmatched in the rest of the series. Materia is one of the best combat customization systems in the series (tied with GF/junctioning in 8), and the OST is phenomenal. Because it's the best one, it got a ton of modding support, and you can completely overhaul your game (google "tifa's bootleg mod pack") to improve the resolution, use custom graphics and sprites for weapons and characters, and load custom soundtracks in based on preference. It's pretty mindblowing. If you want to experience 7 as it was at launch, you can pick it up from steam and just play it through. But if you want a pretty damn polished game that still (roughly) holds up, definitely look into modding it.
6 is a simpler game from a simpler time. It's still got characters, and it's one of the first Final Fantasies to put *a lot* of emphasis on them. Optional characters abound, and you can learn a ton about your favorites through detailed backstories in side areas. Unfortunately, a lot of the characters are stock archetypes, including the villain, and that comes across a little cheesy and over-dramatic in some sections. The combat is restrictive for most of the game, and some characters get to use "abilities" that make them incredible for certain sections (gau is amazing early on, for example, and he can start dealing massive aoe hits before anyone else can, but he quickly becomes pointless as the other characters catch up and pass him by). The soundtrack is great, as always, featuring the single best Final Fantasy song by far [https://www.youtube.com/watch?v=A2f-SeMxpkE] and a strong contender for the best video game song ever. Definitely worth a playthrough, although certain elements are guaranteed to get on your tits.
8 is a contentious one - it's the first one in the main series to deviate from a direct save the world plot, focusing mainly on a love story. It evolves from that, as it obviously must, and expands to contain a very colorful cast of memorable characters and quite stunning FMVs. The soundtrack is great (recurring theme; Nobuo Uematsu is a god among composers), and the combat customization is on par with FF7's (far and away better than most games in the series). The plot of 8 is its weakest point, however - it becomes a confusing mess around halfway through and never really recovers. The rest is generally good enough to keep your interest throughout the remainder of the game, though. 8 also gets a special mention for Triple Triad, which is one of the most baller minigames in any game (and especially in Final Fantasy).
9 is often overlooked - unlike 6, 7, and 8 it didn't do anything to expand the genre or bring new things into the mix, it was simply a retread of older ideas that had proved popular. The characters are fun, the setting is great, the music is... alright (not as good as previous 3, unfortunately). I would recommend it after any of the others that precede it.
10 is where things start to go downhill - graphics and voice acting were focused on at the cost of characters, story and gameplay. The free-roaming world is sacrificed for a linear hallway, and majestic airships are replaced by a drop-down menu where you can teleport from place to place. Voice acting is introduced and serves absolutely no purpose, often detracting from the quality of the game instead of enhancing characters. Only a couple characters had VAs that matched them, and the rest were an absolute trainwreck (looking at you, Yuna). Would not recommend as a starting point of the series, although it's "a better game" than the super early ones. It's also got a pretty coherent anti-religion theme, which is bonus points from me, but the rest of the game wasn't quite up to snuff.
Past that isn't worth mentioning. Two bad MMOs don't even need to be touched on, and 12 and 13 have the same problem 10 did, just worse - unlikeable characters, blander story, less active combat driven by AI party members, and bad voice acting. 15 is ditching the linear hallway bullshit to go open world again, so it has the potential to regain some of the glory that's been lost over the last decade or so of bad, bland Final Fantasies.
On that topic, though, the super early ones aren't worth your time. 1 is the definition of generic and barely has a story at all (and literally no characterization), 2 is awful and has one of the most useless and counterintuitive upgrade systems of any game ever, 3 is basically 1 again but with more classes, 4 is the first one to feature characters with personality and what could even be considered a "character arc" (none of it is all that great though), and 5 is... halfway between 4 and 6. It's got characters, some are neat, they tend to be focused on more than in previous titles but not as much as in 6. It's an alright game; if you really, really loved Final Fantasy 6-8 and played 9 and 10 as well, 5 is probably your next stop.
For the first game in the series to play, pick from 6-8 though. They've all got heavy emphasis on characterization, exploration and backstory. Combat in 7/8 is better than in 6. Music is pretty great regardless of your pick. 7 and 8 are available on steam, 6 had a great GBA port floating around if you still have one of those (or, yknow, a computer). All pretty easy to pick up and enjoy.
Real talk, though, 7 is the best one. It has plot holes, it has weaknesses, but realistically, every game does; 7's are pretty minor if you start comparing them to other games in the series. It does more things better than most of the other games in the series, and its twists and its villain were so groundbreaking that they're still iconic today, even to people who've never played them. Sorry fanbois of 6, but nobody really gives a shit about kefka or locke 20 years later
