I liked the original quadrilogy, but it's the only
Phantasy Star series I liked. My reaction to PSO was even starker than my reaction to Final Fantasy's shift to real time: "No. I do not want an MMORPG, if you want to use the license for something at least use it for something in the spirit of the original games."
I actually like all of them a fair bit. I liked PSI the least, but even it was a fun ride story-wise - the gameplay was more of a "That was fun, let's never do it again" for me. By contrast, believe it or not I enjoyed PS2
's gameplay: it was hard and brutal, but when I picked it up again after some time (look, I first played it when I was five...), I was up to the challenge and a little grinding didn't bother me. And PSIV was PSIV; absolutely stunning in all respects.
So what about PSIII? Believe it or not, I enjoyed that too, though it helps that I only played it years after the Genesis era was over. I give the writers mad props for trying to pull off the impossible with the limited cartridge space they had available, and I honestly think they did pretty well at doing the four-paths thing.