Phantasy Star Online has been replaced by Monster Hunter, for my family/friends and myself.
Monster Hunter took the concept and built upon it in so many ways. Now granted I've only played PSO, PSU, and PSP2 so if any of the newer Phantasy Star games have changed things up then I'm unaware.
In MH, each weapon type has unique roles within the group. In PSO it really didn't matter who used what because while different weapons had different animations, they all did the same thing. In MH the Hammer is smashing the face in for a KO, the Horn buffing the party with songs, the Longsword trying to off the tail, the Dual Blades applying as much status effect as possible, the Insect Glaive trying to mount. I've played Longsword, Bow and now Horn and each time I feel like I'm bringing something unique to the team.
It took away leveling up and made the game skill based. Now granted you still have the progression of stronger weapons and armor, but if you're good (or bad) enough at the game none of that matters. Combat feels so much more "alive" or exciting because it plays more like an action game than a by-the-numbers RPG. If I couldn't beat something in PSO the solution was to go level up. If I can't beat something in MH, most of the time the only solution is to practice and get better. Which, let me tell you, gives such a sense of accomplishment. Characters have weight to them, attacks have openings to learn, and when you finally learn it and it all comes together it's wonderful.
I love that the focus of MH is pretty much "boss fight: the game". PSO's running from room to room, killing mindless drone after drone seems so drab now in comparison. Every quest in MH, within 1-2 minutes of starting turns into an exciting on-your-toes boss fight. It's as if the filler has been removed from the formula.
You interact with the monsters so much more in MH as well. In PSO you'd just hack away at a boss with zero interaction as they went about their way. In MH the monster reacts to your actions by flinching, falling over, having their parts broken, getting trapped/flashbombed, running away when hurt, get sloppy when out of stamina. The players actions are having noticeable effects on the monster.
Monster Hunter just comes off as such a better polished product nowadays. It's one of, if not the most important franchise Capcom has and they have a lot riding on it. It's their baby and they take care of it well. I can't say the same about how Sega handles the Phantasy Star series.