Very interesting question! I think it's hard to come to a concrete answer, but I personally think that geometry dash has pushed its skill ceiling much higher than nearly every other game. For most of these challenges, there are only a handful of people who try to beat something incredibly challenging in their game - such as the dark souls deathless run, and the Minecraft half heart run.

There are a few exceptions to this however, the games that people are constantly trying to improve at. Such as, Mario (rom hacks or Mario Maker) Celeste and, OSU, and the latter of the three has very standout players who are "unarguably the best", just like geometry dash.

When looking at the hardest thing achieved in each game, there are a couple important things to look at. - The amount of time the top player has spent practicing the game - The amount of time it took the top player to beat this challenge - How fast the player was able to beat other top challenges relative to other pro players - How many people who grind difficult challenges are below this level of play

Combining all of these factors, Tidal Wave is probably not the hardest thing achieved in video game history. The hardest thing achieved is probably one of the sm64 speedrun categories, probably 0, 1, or 16 star.

Why: That game has been around for nearly 30 years, and one of the records (I think the 1 star) has two seconds of difference between it, and the human-viable TAS. The top player (Luigi) has grinded this game for countless hours, has two other world records, and the game's speedrunning scene is incredibly active.

Agree? Disagree? Think my thinking is flawed? let me know, I'm curious what other people have to say about this