As someone who has played most of the classics, I'd recommend starting with any of these: IV, V, VI, VII, IX, or X. Each of those stand out either thematically, mechanically, or as fan favorites.

While the series was turn based (at least main series offline), the battles mechanics could vary greatly. Final Fantasy I, IV, and IX have the party members locked into specific battle roles. FFX also starts out this way but can give you more options by late game or, depending on the version, after some quick grinding for levels and special items. FFII, VI, VII, VII and XII are more open about how you set up your character's roles in battles. FFIII, V, and FFX-2 have "job systems" where the roles are predefined, but can be swapped and new roles earned over the adventure. FFIV also is the only mainline game to have a max battle party total of 5, and FFX was the first to let the active and reserve characters be swapped mid-battle to change up how strategies would work.

Thematically the stories could sometimes get different, too. Most of them boil down to some bad guy wanting to take over/destroy the world with some comic relief enemies thrown in the early game, but a few of them can get bleak. FFX does it often.