FF15 is so different from the rest of the series that I'm going to tackle this question as if you were a complete FF beginner. I'm going to give you my thoughts on each game more or less and then you can decide from there.
FF1 is a great choice for beginners. It has been remade a million times and Square has smartly made the game easier and easier with each remake as to incentive casual players to jump in. The GBA, PSP and mobile versions are all super easy and accessible for new players to turn based combat. You pick what jobs your party can have at the beginning of the game so there's an element of customization there if you're into that.
Avoid FF2 and FF3. FF2 is not that great a game and FF3's pacing is fucked.
FF4 is another great choice for beginners. If you're worried about difficulty tackling these old school rpg's, then this one might got you covered. The SNES version is suuuuuper easy. This is the first FF to put some actual appreciable effort into making an actual story. It went as well as you can expect for a SNES game from 1991. I tease but the game does have interesting characters and memorable moments, it's just very tropey and predictable. Cecil is one of my characters in the franchise. This is also the first game to use the patented active time battle system. This is also when the music got good, as in real
good. Can't go wrong with this game as your next choice, it laid the groundwork for the masterpieces the came later. The DS, PSP, SNES, GBA and Steam versions are all good.
Now FF5. In my opinion, this is Sakaguchi's magnum opus. Instead of making an extravagant story of convoluted proportions, Square decided to focus on the gameplay. The job system implemented in FF5 is the best gameplay mechanic of the franchise, giving the player essentially endless possibilities accessible at all moments. If FF4 was when the music got real good, FF5 is when the bosses got real good. Instead having a bigger number than the enemies, FF5's bosses actually require critical thinking and strategy. If I'm making it sound like FF5 is hard, don't worry cause it's not. FF5 hits that sweet spot where it's not braindead easy but it's also not incredibly difficult. It's easy enough where you can progress with basically any job, but challenging enough for the player to feel like they're making relevant progress. In other words, it's the best Final Fantasy. It also has the best
chocobo theme. I could say more about FF5, like how hilarious and charming the story is, but I don't what to turn this into a FF5 gush post.
FF6. What can I say about FF6? It's a masterpiece. This is Nobuo Uematsu's magnum opus as it has the best music of the entire franchise. It also has the best characters, story and villain of the entire franchise. If you play FF6 next you might not enjoy the rest of the series that much because no game hits the highs FF6 does.
If you can stomach the awful visuals of FF7, it's a great choice for beginners as well. FF7 is on the easy side, not as easy as FF4, but still pretty easy. The characters are really charming as well. Cloud, Barret, Aeris and Tifa especially are really awesome to get to know. This when the games start to get really long too. FF6 is around 30 hours and FF7 is more around 50. It also has the coolest limit breaks so there's that.
I hate FF8. I hate almost everything about it. I hate the junctioning system, I hate how the enemy difficulty scales with Squall, I hate the overall aesthetic of it. I am your stereotypical internet FF8 hater that you hear about so I'm not really the best at talking about FF8. I love the card game though.
FF10 is awesome. The story is a mess and the characters are pretty bland all around, but the gameplay is really fun. It's fast paced and being able to tag party members in and out gives the active time battle system a breath of fresh air. And the music? This music is so good it gives FF6 a serious run for its money. This game also serves as a great link between classic FF and modern FF. If you can turn your brain off for the story there's a lot to enjoy with FF10.
FF13 is yikes tier. Easily the worst story in the entire franchise and the worst party of the franchise as well. Lighting herself is cool and an iconic design, no doubt, but the party she surround herself with is the actual worst. I hope you like reading codec entires because your hopes of understanding the story is 0 if you don't take the time to read literal wiki pages. The gameplay is very hit or miss as well. In theory the gameplay system of having characters basically change jobs in the middle of battle sounds awesome, but the execution was sloppy. Random encounters are a drag because the take forever. Avoid at all costs.
I haven't played FF9 or FF12 so I can't help you there. But they're probably good.
