I tried playing GT3 in PCSX2 once, and it was basically unplayable because the graphics were so glitchy. That was years ago, so maybe it's improved by now. You can always play it on a PS2, though. Personally I think GT3 is the best in the series. To me, GT and GT2 were impressive considering they were on the PS1, but they were limited by the PS1's hardware. GT3 was like a progression of that, but without the PS1's graphical limitations. GT3 alone was a big part of the PS2's success, I think.
I don't mind playing it on my PS2 but I'd rather enjoy the graphical upgrade that comes from emulation. I'll give it a shot as I can't see them not improving the emulation quality given how big of a game it was for the PS2.
I think the question is a bit too big to say which is the best, because there is a 100 different angles to take on which one is the best. But... If you love plug and play racing games GT2, GT3, or GT4 will do. They are all spectacular you can't go wrong with any of those 3. Post GT4 (PS3 era) it gets harder to rank bc they are very online and download heavy. Lots of fun when new, but as they age, the backlog of downloads and missing online content can make the game feel incomplete.
GT1 will always be my favourite but can't fault GT4, especially with a set of component cables and the resolution up to 1080i (only works on the US NTSC version).
To this day I can't believe how good GT4 looked on PS2. I mean if you look at some of the earlier PS2 games then play GT4, you wonder if they are even on the same console bc GT4 is so far ahead graphics wise.
For those who were interested in this version (GT5 magadrive), know that it is a hack of the game FUNCARRALLY; They follow my videos by disassembling and opening the box. Rom Downloadable in the description of the second video.
I think GT, GT2, GT3 and GT4 are all "the best". I can't stand online stuff. And yes, you can compare the graphics of GT3 and GT4 and be amazed that they both run on the same hardware!
Considering it had the 1080i mode also! My favourites will always be GT2 [scratch & sniff discs ftw!] & GT4 ... according to PSN stats though, I spent almost 85 solid days playing GT5 ... I would probably have spent more on GT6 had it not arrived so late in the PS3 lifecycle! GT Sport on the PS4 has just not captured my attention so far, I miss many of the old tracks and vehicles!