If you want to stay old school, IV and VI (both of which are arguably my favorite games of all time). VII through X all have something to recommend them (with VII being a must play), but idk if you have a PS2. XII and XIII are, IMO, acquired tastes since they diverge from the FF formula.
And then there's Kingdom Hearts (and the bazillions of handheld titles that are somehow canon) if you want FF meets Disney with the most convoluted lore ever.
I'm also gonna have to give a cautious recommend to Final Fantasy III, especially if the job class system is what you're digging most in V; it was initially developed in III. And if my opinion counts for anything, while the MCs are blanks, I think III has some of the most distinctive world-building and coolest history in the series, and it has
the most underrated FF villain in Xande, who I love to bits and then I love the bits to bits.
So why's it a cautious rec?
Because it'll smack a gamer down harder than a Mack truck, that's why. FFIII is, bar none, the absolute most painfully astonishingly groin-shottingly
hard game in the series. Power-levelling, nothing; III forces you through entire dungeons with status ailments, has more than one battle that can be rendered unwinnable without a very specific job combination, and most infamously, the final dungeon is actually the final
three dungeons in which the random encounters are bosses, the bosses are nightmares, and you
can't ever save.
So, what am I playing right now?
...uh, Final Fantasy III. I just defeated the Djinn. I'll put up with any amount of game rage to hang out with Xande.