1. PS2
2. PS1
3. PS3
4. PS4
The PS2 is hands down the best playstation console in my opinion; it has an amazingly vast library of excellent games and was maybe he only console library that continually surprises me when I think I know about every worth while game in it. In terms of nostalgia this one ranks high as I received the PS2 for XMAS in 2000 when stores could not keep them on shelves and they were being flipped on ebay for double their MSRP. I was shocked my aunt was able to get a hold of one and it was among the best gifts I've ever received.
The PS1 also has an incredible lineup and is probably the best system for JRPGs in terms of quality and quantity. The PS1 also has a lot of sentimental value to me, not because I owned one from the get go, but because it has so many excellent games and franchises from the mid to late 90s on it.
Regarding the PS3, it also is a great system, but at least for me it doesn't hold a candle to its two predecessors in terms of the quality and quantity of its game library. However, in terms of amount of time I've spent playing games on it, the PS3 surprisingly may have the PS2 and PS1 beat, mostly do to games like Oblivion and Skyrim and the crazy amount of time I spent playing both.
I rank the PS4 last not because it isn't an amazing system or doesn't compete with the others, but because I feel like it is too soon to properly rank it anywhere else. I bought my PS4 about 2-years ago and while I have gamed heavily on it, I'd like time to tell where it ranks among the Playstation consoles for me. If my experience so far is any indication, it will likely rank higher than the PS3 someday, but I doubt it will catch up to the PS2.