Wow, This is so tough to choose between especially PS3 and PS2 but nostalgia more or less settles this list for me.
#1 PlayStation 2 - Playstation 2 combines innovation, childhood memories and one of the most dominant and loaded libraries in gaming history. It is one of the most perfectly rounded and essential video game consoles ever and not only launched so many of the greatest franchises in video game history (God of War, Sly, Ratchet and Clank, Kingdom Hearts, Shadow of the Collossus, Jak and Daxter) to name a very short few it also dominated in pure 3rd party depth. It's not only the best playstation console ever, it may be the best console ever imo. Dualshock 2 was also never beaten and Sony took multiple steps back with the controller. PS2 is the console I grew up through almost my enitre life with.
#2 Playstation 3 - Yet another amazing huge library. It's as close to the PS2 as we can get. It's truly an amazing console and right up there with PS2s legacy. All the franchises seen amazing games in this generation and it was a pivitol moment in Sony's Legacy.
#3 PlayStation Portable - Not on the list but is a essential and amazing handheld.
#4 Playstation 1- The one that sparked it all. With amazing unforgettable classics like Parappa, Tomba, Crash, Spyro and endless others. It's 3rd party support was phenomenal and it truly spanked its competition.
#5- Playstation 4- To be honest sony has dropped the ball multiple times with the PS4 but it's still an incredible system still. I just wish some of the major franchises would have had been made more and it seems this library doesn't hold up to the previous 3 units. Their are a few amazing exclusives but not PS3 caliber library wise
#6 PS Vita- The biggest blunder in Sony history. A complete failure that doesn't get the heat it deserves. It's very one dimensional in it's library and i'd put it at the bottom of my list.