The A.I. in GT1 were outright ruthless...and a bit stupid. On the first corner of Autumn Ring they would miss the braking point and slide right off the track. If you were in the way, you were likely to get taken out.
In GT2, the A.I. were a bit more forgiving, but on Rome Short they seemed to forget how brakes work. You would likely get crunched into the wall on the 1st tight corner. On Grindelwald's reverse circuit, sometimes the AI would turn immediately left and rub against the wall for a good 4 seconds.
In GT3, the A.I. began to calm down, but their pit-stop techniques sucked...HARD. Grand Valley is a common place for an A.I. to violently fishtail and eventually spin out. On Seattle Circuit they would either brake too early on the jumpy hill section, or brake too late and end up smashing the wall.
In GT4, the A.I. were completely mindless and annoying. They just...followed a scripted line. If you got in their way, you were likely to get spun out.
The AI in GT5 acted very unpredictably. On most tracks, they slowed up if you were on their tail or on their side, nearly coming to a complete stop sometimes. However they swipe the walls on Daytona and Indy Super Speedway due to carrying too much speed.