Phantasy Star is a good game if you bear in mind that many RPGs were pretty barebones back then. It feels like Dragon Quest I/II but with a sci-fi setting and 3D dungeons. The music of course is good, and more importantly, very unique. Bo's music just seems to perfectly capture that space colony feel.
Phantasy Star II is a classic and very much worth a look. It has a story that was perhaps ahead of its time. The only potential downside is the difficulty. Getting through dungeons tends to be a war of attrition and level grinding is a must. Like with the first game, the music is great, and with that same sci-fi kind of sound.
Phantasy Star IV is the most accessible of the original games. The pacing is more consistent and the game is easier than the others. It has nifty comic book-style cutscenes and the music is pretty good as well (no Bo though), albeit with more of a rock'n edge versus the first two games. It's a good, solid RPG.
I have Phantasy Star III, but I haven't played it yet. I only know that it's considered to be the black sheep of the series since it apparently wasn't developed by the same team. I don't own and haven't played any of the PSO/PSU games.
So for me, I'd say that Phantasy Star II was the most memorable. Something about the atmosphere of that game has stuck with me, and I only first played it a few years ago.