Which PS system has the best first party lineup, and why?

[Leynos]

PlayStation 2, but PS1 is a close second.

You have your bigger hitters like God of War and Twisted Metal, Jack and Daxter. Then they put out Ratchet & Clank. Mark of Kri, Wild Arms. Sly Cooper. For better or Worse, Killzone. Made that Jet Li game. Published Rogue Galaxy. Dark Cloud 1-2. Ico. Extermination. Okage. Kinectica. Frequency. Drakan the Ancient Gates. Everquest. Hot Shots Golf. Soccom. Jet X2O. War of the Monsters.The Getaway. Primal. Arc The Lad. ATV off Road. Eye Toy games. Syphon Filter. Champions or Norath. Genji. Shadow of the Colossus. Ape Escape 3. GT. Singstar. BUZZ. A bunch of sports sims.

Even ignoring the vast 3rd party library of PS2. If you just wanted a Sony-published game. They had a genre for you. They covered almost anything. Variety and mostly quality.  In Where PS3 era, they mainly just did shooters, and PS4 era became sad dads in forests, and the PS5 just Fart gas.