The correct answer is GT2. It was ground breaking, unrivaled in it's time, fun, tons of content, great soundtrack, the now classic signature menu themes and sound effects, Bleemcast!, racing modifications, great variety in specialty races (wagons, convertibles, etc.) And unlike some of the others here, it's still fun to this day because it's the perfect blend of arcade/sim on a controller. I played a bunch of it not that long ago on my RGB modified CRT TV. This game had an effect on who I am today. And I went through the headache of importing a R32 GTR 16 years later after falling in love with this series,
because of GT2. GT1 was good and all but it was a proof of concept.
Edit: Oh also, the "100 skylines" argument is such a cop-out because, as I've been friggin trying to tell people (on GTP) for 15+ years now, it probably took them all of two seconds to add another one. The vehicle model is done. Just copy and tweak it. Does it suck that they pad out their car number with that? Yes. Did the Skyline add a year onto it's development cycle or cut into their budget? No. If they didn't add those, would they have added 100 other cars instead? Nope.
Then I'd say GT4. Play it with a wheel. Change your oil. Do laps on the Nurburgring. Stfu. GT3 was good for a launch-ish PS2 title and obviously had the nice visuals for the time but that's about it. Was more like a demo. Dreadful car selection. The handling/cars felt very stiff.. like I was driving around a block of wood. I'd rank GT6 above GT3.
I haven't finished the podcast yet (I listen while driving for work all day) but I would've liked to hear them each state their age and when they played their personal favorites. I would've been about 12 (turning 32 this Friday?) when GT2 came around and I think there is a correlation between the two.
@Jordan probably prefers GT2 because of his age and time in his life when he played it. I'll bet the people who like GT3 are a few years younger.
Anyway, to commemorate this argument as old as time, I'll embed these GT videos I made almost 7 years ago? GT6 wasn't out yet I guess. They aren't very original but I had fun throwing them together and they've been unlisted online ever since. First public appearance!