I have it down to four to pick from.
AO 2012 = as he matched peak level Djokovic all the way and without doubt should've won the match, that missed backhand at 4-2 up cost him dearly.
AO 2014 as he was expected to win and despite injury i think he mentally panicked and broke down. This match was to equal Sampras and would've had everyone making him certain to pass Roger. Despite going on to win the French , this defeat was the seed of the decline between 2015 -2016.
AO 2017 - An epic that he had the initiative and let it go. Yes Fed's Neo backhand and Rafa was gassed after the epic semi final war against Dimitrov (how else can we explain that 6-1 3rd set) Yet Rafa was 3-1 up in the 5th and should've seen it through. This was the first loss that determined that he wouldn't catch or surpass Roger.
Wim 2018 - I believe this was ultimately the match that cost him GOAT status. He should've won the match so badly against Djokovic who was still on the comeback. This again highlighted why Rafa's weak serve costs him so much. Had he won here he probably would've seal #1 for the year and gone past 200 weeks at the top. This of course generated the momentum for Djokovic to rise back to the top allowing him to regain confidence.
The tragedy here is that all four matches Rafa would and should've won if he had a stronger serve to consolidate his breaks of serve. The last two especially i think have cost him GOAT status as they would have given him more slam diversity and weeks at #1. As we stand at the moment of course he can still do it but we are the critical stage now where the quest will be over if he doesn't win at least one of the next two slams. The next big injury will likely force him to retire. I hope he can do it and overcome Djokovic for the sake of men's tennis , but it's not looking good.
Unfortunately i think Nick will blitz him off the court in 3 tomorrow.