(Not sure how familiar you are with FF games so sorry if I over-explain stuff you already know)
What order doesn't matter in a story sense since every numbered final fantasy takes place in a completly new and different world with a new story and new characters. What connects Final Fantasy games are more like tropes (almost all games have summons, chocobos, moogles, a guy named Cid).
Every game has slightly different battle mechanics. Some are more traditionally turn based (meaning you can take your time in battle) while others use the ATB (active time battle) system which basically means you have a lil loading bar for when you can make your next move, the recently released FF16 is the most different gameplay wise since it is a more traditional action game.
The different games also have different vibes/themes when it comes to the story and setting? In the mood for a story involving a corporation in a industrial/semi-modern sci-fi city causing ecological disaster by draining the earth of it's life force? give 7 a shot! want a more traditional high fantasy setting with castles and silly knights? give 9 a try.
It really all comes down to what variety of vibes you wanna go for when picking a FF game. My recommendation is to look up the characters of each game (do any of them have "the spark" for you?) or read a quick spoiler-free summary of the story or perhaps look into what the actual gameplay is like in this one since the games are pretty long so you better like it :P
Hope this was somewhat helpful