2009 Australian Open Final. I remember the 2008 Wimbledon tournament, and I remember how much of the coverage leading up to the final was focused on Nadal. He felt like a favourite from the beginning.
Down Under however... Nadal had never before reached a hard court Major final, and was going up against a Federer who had won 8 hard court majors without losing a final, and was in great form. There was also the question of whether Nadal would have the strength to carry on after that marathon against Verdasco.
The first 4 sets looked like they had gone Federers' way. Despite being 2 sets a piece the scorelines suggested that Federer had won his sets more convincingly, while Nadal won his on a Federer double fault in the first, and a tiebreak in the 3rd. Federer was also ahead on points.
...then Nadal wins 6-2 in the fifth. Nadal had won his first Major title on a hard court, in his first hard court Major final, and all of a sudden the clay court grinder from Mallorca had won 3 of the 4 majors, on all 3 surfaces, and all against the most successful player of this generation.
When Nadal won Wimbledon it looked like he would become world #1, dominate the tour, and perhaps go on to win close to double figures in Majors.
When he won in Australia... the world realised he could dominate everywhere. He proved he could win Majors on grass, hard court, and more importantly he could win Majors on clay.
In my opinion it was this match that opened up the GOAT debate that little bit more.