The first Phantasy Star is good if you can stomach the Japanese Wizardry style of the game. It's really neat because it's a ~strong female heroine~ in an old JRPG which didn't happen a lot and it plays pretty okay! If you played early dragon quest, you could make your way through it. I don't think a lot of people care for it becacuse it's too outdated, but the same goes for the second game
Phantasy Star II is less accessible, imho, despite becoming more of a JRPG, although it's still uh… How to explain
It's got a lot of archaic mechanics and it's very much an old fashioned prolonged, sometimes aimless JRPG and it kind of takes the most archaic from first person AND third person console RPGs and combines them into a goofy product. still good if you can get into that sort of thing.
Phantasy Star III is when the gameplay gets a bit more modernized and it's on a Final Fantasy IV level of "oh I know where to go and what to do" and despite the interesting idea of playing through a genealogy, it's the big black sheep of the series. Not particularly great?
Phantasy Star IV is really good and it's as modern as a JRPG could be. It's super comic-booky and uses panels to form cutscenes and it does this all the time so it comes off as high-budgety. The battle system also has programmable macros and that's pretty neat. Best Phantasy Star game really.
I haevn't played these in yeeeears though
I like them all in a sense so I'd recommend if you're patient, yeah, play them all. But I've got a soft spot/guilty pleasure for some really old console RPGs like the PS series.
If none of that sounds appealing to you, only play Phantasy Star IV which is by and far leagues above the others in accessibility and enjoyability for most game players. It's just really charming.