Good poll. 👍 I voted GT2. It was possible to take lots of vehicles to the World Cup in this game which would get slaughtered if they appeared in other games.
I took a regular C4 Corvette (race kit attached) for instance. 650 horsepower if I remember correct. [R] Viper GTS. Honda S2000 GT1 (500 horsepower or thereabouts), Alfa Romeo 155 Touring Car (544 hp).[R] '92 Mitsubishi 3000 GT (676). Venturi LM. Vector LM. Renault Espace F1. Some others I'm (no doubt) forgetting.
In most cases, GT1 cars were present. Granted, some of the cars I drove would do okay in GT4's World Cup, but I doubt they'd be able to win all of them.
I voted GT3 because it's all I have to compare against GT4. The Beginner League GTWC race is easier than throwing a cat at a barn door...from a few millimetres away.