This is tough; the PS1 is one of my favourite consoles of all time and was a big part of my childhood.
If I had to narrow it down to just one game I'd probably go with Crash Bandicoot 3: Warped, but I also really loved Crash 1 and 2, the Spyro trilogy, Croc, Crash Team Racing, Soul Reaver, Wipeout, Doom, Medieval...
Such a fantastic system.