Back in the day when you actually had to leave your home in order to play: Defender.
For the short while I borrowed a PS2 it was GT3 hands down.
And when I'll get a dirt cheap PS2 (hopefully 4 Christmans), cause $600 4 a PS3 is way out of my league, I think it'll be GT4.
My absolute favorite though was a little thing called Galaga...
God the money I wasted on that thing...