PS3 is the strongest, in terms of hardware (duh).
The PS2 has a completely insane backlog, since it barely had any competition during its peak years (Gamecube? Xbox? Hah!).
As for the PS1.. well, it had some great games and apparently a really good audio chip. That's as far as my comparison goes, friendo. Its games have aged pretty poorly, though. 3D models from that era are impressively awful (see FFVII).
It's important to remember that the game industry was very different back then. Microsoft wasn't a competitor in the home console market, and Nintendo mostly catered (and still caters) to a different demographic.
EDIT: Oh, I'm sorry. Did you want a meaningless and hopelessly arbitrary opinion on which one is "better" based on some completely arcane parameters?
Fine, the PS2. It kept kicking ass all the way to 2008.