GT1 is the most important of all because it was first. It has unique features not seen in later games which are pretty cool. If you've never played it and enjoy GT5, you might be too jaded to be able to appreciate GT1, but otherwise any long term fan of the series will acknowledge its greatness, though few still play it.
GT2 was a vast improvement over GT1, though it was rushed into stores and left somewhat unfinished/unpolished. More cars, more tracks, rally mode, better pyhsics.
But I see where C-ZETA is coming from. Too easy to win races with overpowered cars and so beating the career mode is a trivial accomplishment, leaving no great sense of satisfaction after the dozens or hundreds of hours invested in playing.
But as a simulator, ignoring any goals about winning races and just looking at the cars and tracks themselves, GT2 is an endless fountain of amazing gameplay. Hundreds of cars with unique handling and performance and a couple dozen races to test them in against a wide variety of opponents, and then multiply that by tuning options.
Or swap in the Arcade disc and let the playstation pick your opponents for you, though even this you can do in simulation mode with the event synthesizer, another amazing feature of GT2.
If you've never played it before, even if you are spoiled by GT5, GT2 might just be good enough to get you to overlook its outdated graphics and interface and spend time playing it instead of other much more modern games. But don't feel guilty if you try it and just can't find anything to enjoy about it. It may be that nowadays GT2 can only be appreciated by those who played it back in the day.