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The PS2, as far as the sheer volume of content goes, is the best. However, I would argue that the PS1 is the more important console. It broke games away from the image Nintendo had cultivated and brought us a lot more variety in both style and substance, not to mention a cooler vibe. It felt like such a huge leap over the SNES for me in terms of what it offered. Suikoden 1 & 2, Final Fantasy Tactics, Xenogears, Soulblade, Final Fantasy 7-9, Gran Turismo, and Silent Hill, among others, were such massive steps forward. Prior to the PS1, gaming wasn't really all that important to me, but its arrival kind of changed everything and turned a time-killer into a hobby.
That being said, I think the PS1, PS2, and PS3 are really close. I rank the PS3 last only because of the death of the JRPG on home consoles, though there were huge improvements in other genres.
