Guess a lot of this depends on how you define great, but the Mk 1 GTI defined a whole genre of cars, so there must be a degree of greatness there!
Whoever had the idea is a true Great! I had a Mk1 Cortina GT around the end of the '60s, which had about 80 bhp from memory and had the same sporty accents, but it was a much bigger, heavier car, so the performance and handling were not close.
Without the Mk 1, none of the others, plus all the competition, would exist.