P3 is the best because of the simulation/rpg mix. In P3 you have to make conscious choices. "Do I grind it out for another night and risk my party getting fatigued and limiting the things I can do?" You balanced the sim with the rpg. It's very hard doing everything in the game and maxing every s link. The amount of choices makes every choice have weight in how much it affects the simulation to the rpg. There's also the fact that there's after school and Tartaros. This allows a nice balance between choosing what activity to do after school and at night. You essentially have two choices a day. In P4 and 5, you have one. People had a problem with P3's fatigue system which forced you to actually weigh choices and they were wrong.
Furthermore, I think P3 had the best slinks. In P4 there's far too many s links that are tied to main story characters. In P5, every slink is tied to the story. It was great how in P3 you had stories like Maya and the mmo, or meeting a dying man in a park. You used these people to grow better Persona through your high charisma for selfish reasons. It felt like your actual characters person was himself a persona, or rather, using a facet of his personality to use people for power. In P3 it's sunshine and rainbows (literally given the games aesthetic).
Now we have P5 and the simulation has been neutered into shit. What really makes it worse off is the fact that like in P4, you go into dungeons only in the day time and can't do anything after the fact due to being too tired. It's been a while since I've played P4 so correct me if I'm wrong, but that doesn't happen in P4. You can do a dungeon and then at night time, go to your job or do an slink. By limiting my amount of choices (slinks or dungeon) you don't really have much of a choice, because dungeon will almost always take precedence. So because you only have one choice per day, the developers are obviously going to make leveling up slinks easier. Maybe I'm wrong, but that's what I got out of my time with P5. The simulation X rpg part was downsized when it was originally the main hook of modern Persona. They opted to take it more in a visual novel direction instead.
For that reason I don't mind that P3 has party AI. The simulation X rpg aspect (the main hook) made it the most fun to play. Maybe P5's simulation opens up better later on in the game. I don't know. All I know is they neutered everything I loved about modern Persona to appeal to people who want to max every slink in one play through.