I'd have to say the PS1. Think about it:
It moved us from 2D to 3D in a matter of months.
It moved us from cartridges to discs by the time it was discontinued in 2001 (Xbox and Gamecube both following the trend.)
It could have very highly detailed models and higher-res images than both the N64 and Sega Saturn could, at the expense of an increase in "jaggies". Seriously, look how good Dino Crisis 2's graphics were:
And the detail in F1 World Grand Prix 1999:
At the time of release, the gaming industry was completely dominated by Nintendo and Sega, yet somehow it was able to fend them off with its own IPs and didn't go the way of the Philips CDI. It was also the only console that could even remotely keep up with the PC. Its Doom port was one of few to contain nearly all of the original game, and came with a tense new OST.
The PS1 introduced us to franchises such as Grand Theft Auto, Resident Evil, and Gran Turismo.
It saved the PS2 from an early death similar to the Dreamcast because the smaller PSOne released in 2000 was being bought by gamers who couldn't afford the new console.
It had more sales than the SNES, Sega Genesis and Sega Saturn combined.
It had very few failures. Apart from the occasional disc laser burning the disc tray in early models, nothing was a common problem.
The PS1 allowed us to save data to memory cards, rather than the actual cartridge. Very few games used a password save.
It lasted 12 years before being fully discontinued, which was after the PS3 had been released.
Finally, the PS1 was one of the first consoles to allow people to homebrew-develop their own games due to the Net Yaroze.
