PS2, and I would say it's not even close. I am basing that on third-party exclusive games. Back in the early 2000's, we had the GameCube from Nintendo, PS2 from Sony and Xbox from Microsoft. All three were actually pretty comparable in terms of CPU and GPU and RAM. You could make the argument that the GameCube was a bit harder for certain games due to the disc storage size; those cute little GameCube discs didn't hold nearly as much data as the DVD-ROM's of PS2 and Xbox, and that does legitimately make certain games impossible to release on it (like Grand Theft Auto for example; you can't expect someone to switch discs every time they cross a certain road or something).
But despite both PS2 and Xbox being nearly as similar as the PS4 and Xbox One, the gulf between them in sales was SO MUCH that PS2 got the vast majority of games. Imagine a world where Call of Duty or Far Cry or Grand Theft Auto was exclusive to PS4 just because it wasn't even worth the development costs of an Xbox One release. That was the PS2 dominance back in the day.
