Like everyone says, PS2 and its not even close. The PS2 had multiple releases of each AAA game, jak had 3 or something, two god of war, two great GT games, MGS2 and MGS3, Ratched / clank, sly cooper, three great SSX games, soul calibur 3, virtua figher 4, Tekken tag, tekken 4 and finally tekken 5. System also saw some of the greatest 90's games, Half Life and Dues ex, quite good ports too, considering.
ICO and Shadow of the Colossus where masterpieces, graphically more advanced then anything PS4 has/had for the time. Black showed what the PS2 could do when pushed aswell, very pretty looking (and sounding) FPS title, great controls too.
PS2 was also a platform where 60fps was a thing, God of war 1/2, ZOE2, MGS2, and many more were 60fps titles, while being graphically one of the / if not most great looking games of all platforms. In comparison PS4 is quite a disaster with its 30fps with dips output.
On PS2 we also had timed exclusives like GTA3, Vice City and San Andreas, which were even more reason to own a PS2.
Devil May Cry one and two also great games.
Final Fantasy was rather great too, SOCOM series had great online gameplay for being a 6th gen console, where online wasnt even really a thing yet. Free online aswell.
Many japanese titles like Klonoa 2 saw releases on the system, JRPGS also being a thing there.
Eye Toy also saw mutliple releases on the ps2, as did many pheripals did.
Multiplatform games wherent bad either, we saw TimeSplitters 2 and future perfect, prince of persia, Tony Hawk games, and much more sport games then the PS4 ever will.
Also, when PS2 launched it had rather high-end hardware (instead of like PS4, low end hw), it was more capable then a Geforce 2 in some ways, at a very respectable price of 300 dollars, and going down in price much more and faster then the PS4.
I remember everyone being blown away by the MGS2 E3 demo.
PS2 came with something new too, DVD player, like the PS3 brought blu-ray. PS4 didnt come with anything really, not even UHD 4k blu ray player in the pro somehow. Support for DTS/DD in some games for a march 2000 console is rather impressive too.
PS4 seems to have more heat/noise problems then the PS2 aswell, aside from laser-problems, the PS2 didnt really overheat and die thanks to it. 2 USB ports on PS2, 2 on PS4, rather lacking for a late 2013 machine, somewhere.
On top of all this, games werent just movies for the most, gameplay was a thing there. Oh and, full backwards compat without having to pay twice for the same games, even the PSX controllers/memcards etc where BC.