That's a tough one. I'd say it's a tie between PS1 and PS2, the former has iconic titles and the latter has a huge quantity of less iconinc but still high-quality titles (Xenosaga, Valkyrie Profile 2, SMT, P3/4, FFXII, Suikoden V, Shadow Hearts, etc.).
The PS4 is a solid contender for a 3rd spot thanks to the many ports, remasters, and new games. If the PS5 has an equally as good library, that's all I could ask for.