PS3 was pretty cool -- for sure innovative. But it definitely didn't "fit in" so well with the rest of the series. It really is more of a side-quest in the Phantasy Star cannon, much like PS Gaiden for the gamegear.
PS2 was definitely the best one IMO. Storyline, characters, all tops. And the challenge level of this one is definitely a case of "Man, they don't make 'em like they used to", even considering the full-on walkthrough that came with the game. I know a guy who destroyed his Genesis because he got so frustrated with the game (he never knew that a hint guide existed).
PS1 was awesome as well, definitely the best 8-bit JRPG IMO.
PS4's polish-level & production level was top-notch, but... just too damn easy!
And you know, I'm pretty damn sure they'd f*ck it up if they try to bring it back. I'd rather let the PS series just go out with a bang. I'd love to get Playstation 2 remakes (parts 1 & 2) in English, but fat chance...