I think people's memories are kinder than time. I booted up GT1 a few weeks back with dewy eyed nostalgia. As soon as I hit the track I baulked at the low resolution, AI, and terrible collision detection. I remember it graphically blowing away every PS1 game at the time when I got it on Japanese Import.
But I certainty wouldn't play it preference to GT5 now.
One thing that is worthing having from this era: The Namco NeGcon (NOT the JogCon with the spinny wheel thing) - this was superb when you couldn't be bothered to setup the wheel and you just wanted a controller. The analogue buttons for throttle and brake had loads of travel. And the twisting motion of the steering was superb. In GT2 they introduced a huge deadzone to the middle for some reason, which killed the sensitivity of it.
http://en.wikipedia.org/wiki/NeGcon
GT4? Yep it was very good for the time. Local LAN play and multiscreen setup should be in GT5. Still wouldn't play it over GT5 though - the physics in particular or less 'organic' and less 'fun' than GT5.