It depends... In terms of the obstacle in front of him, Roland Garros was the hardest. In terms of the length of time it took to get the chances and how few opportunities he got, the Olympics was the hardest.
In Cincinnati I think he just genuinely, by his standards, was not that great. Forget Federer - Roddick, Murray, Nadal, these aren't Kohli or Melzer-like bizarre upsets, he'd get to the latter stages and then just get beat by other top players. 2011 and 2015 Roland Garros are the only really comparable ones.
Looking at it that way, it is sort of the hardest in its own way. Also makes the 2023 win one of the best of his whole career. The 2018 one was so weather-impacted it reminds a lot of his 2016 Roland Garros win.
So, yes, I think you can make a case for any of the three

You cannot argue he was not good enough to win Roland Garros because 1 man stands between him having something like 10 titles there. For a long while, you sort of could with Cincinnati.
I still go with Olympics though. That is what makes that tournament so amazing. You get 1 shot, and then you have to wait 4 years for another one. Some could argue if you get more chances and keep failing that is tougher.
I'd go the other way. When you get so few chances and want it as bad as he did for as long as he did, that is a crazy amount of pressure that builds up.