I really can't decide because they each had their own specific strengths.
GT1: The beginning of it all. The game that introduced sim racing to the masses and changed many lives (including mine) in the process.
GT2: An iteration of GT1, including better graphics and physics, and introducing several tracks that would later become staples of the series (and a few that desperately need to be resurrected - here's looking at you, Red Rock Valley).
GT3: When GT went back to basics for the PS2. Sure, there was less content than GT2, but what was there, was drenched in atmosphere, with truly eye-popping graphics for the time.
GT4: GT3 on steroids. It took the very solid base that GT3 had established and built on that, with more cars and tracks than any other racing game before
GT5: The first "modern" GT, bringing it into the online era. Revolutionary new online system (when it worked), stunning graphics (mostly), and mechanical damage to add to the difficulty of races. Also, Red Bulls on Daytona.
GT6: Refined online system (when it worked), great new cars and tracks, and later on, the Course Maker, which allowed people to make incredible new tracks and recreations.