1 is pretty old by today's standards, and 2 really improved on it even on release.
3 is, IMO, the best one and is probably in at least my top 5 JRPG's on the PS1. It is wonderful and a classic. 4 is also very good, has some amazing music and intriguing narrative.
5 (Dragon Quarter) changed a lot of stuff and I found not for the better. It doesn't feel like Breath of Fire to me, and for a lot of people is kinda the black sheep.... at least it was until 6 came out and was a mobile fucking garbage game.
i liked dragon quarter to be tbh. probs my favorite of the series. ---