It's probably impossible to compare a game like Donkey Kong to a game like osu!. They're pretty different.
I agree. However, my point is that reaching the 99.9th (or even just 99th) percentile in osu!'s active community is, no matter how you argue it, more difficult that reaching Donkey Kong's 90th percentile. This doesn't necessarily mean one is more difficult than the other; I'd say that reaching the 99th percentile in Donkey Kong is of comparable difficulty to reaching the 99th percentile in osu!.
From what I can see, osu! is basically Elite Beat Agents or Donkey Konga, on crack. To get good at it, you play it over and over until you memorize the level and get fast/accurate enough to not make any mistakes. Nothing ever changes, it's always the same every time. It's pure memorization and quick reaction time. Looks like you have to have really quick fingers and mouse movements to do well at this game. Just watching it is pretty cool, it looks really tough.
Like I said before: There are tens of thousands of maps. This makes it hard to use memory to your advantage. The reason why I say "somewhere around the top 50" or "about in the top 60" is because since there are so many maps and such a variety of them, it's difficult to rank players. Back when there were only a couple hundred maps, the most accurate way to rate players was to have the "elite" players play them all, and the player who had the highest overall score at the end would be considered the best. However, nobody actually expects you to play every single map anymore, which makes that form of ranking impossible and inaccurate.
What if osu! had random elements in it? As is, it's 100% repeatable. You memorize the stage and execute it. But what if there were a few random circles that appeared during the stages that you couldn't predict? Or if a stage lasted 2.5 hours. Would you still be able to get such high combos and 100% a level?
I don't think random elements would make osu! much more difficult. Assuming there was a way to make a program give randomized versions maps which also aren't totally illogical, I don't think it would really make a big difference. This is the main way Beatmania players play their game. However, because there are tens of thousands of maps, the best players generally only play them a few times to put up a monster score, then move on. While memory is a factor in osu!, the game requires you to gain skills which apply to all maps much more than it requires you to memorize patterns which only apply to one. A good example of this is geometric patterns. For some reason, people naturally do semi-circular motions when they try to hit square patterns. This makes learning to hit circles in square patterns almost impossible, and some players who have played for years still can barely hit them on any map, no matter how many times they try to play it. Memory just doesn't cut it for this game. Even if it did, a "good" player who can get a good rank on a map with 1,000 plays will have 1,000x the less good ranks than someone like Cookiezi, for example, who only has to play most beatmaps a single time in order to get the best score on it.
Would fatigue become a factor?
It already is a factor. Moving your arm, hands, and fingers that much, even in short bursts, can be extremely tiring.
xi - FREEDOM DiVE (Nakagawa-Kanon) [FOUR DIMENSIONS] This is the best player in the world, and he has by far the best stamina in the world. He now has a 99.97% score on that beatmap. Only around 30 or so people have even passed it, and nobody else has reached a full combo on it. The reason why is because of fatigue more than anything. However, the biggest examples of what you're looking for are these maps, which are titled "marathon maps":
http://osu.ppy.sh/b/156352 http://osu.ppy.sh/b/112922The first one averages on a pretty low difficulty (though it still gets pretty hard on some parts), however it's nearly an hour long and requires around 6500 key presses. The second one is just as hard as any (well, just about any) crazy difficult map, except it's nearly 18 minutes long and requires roughly 4,000 key presses. However, most people would probably agree that the four-minute beatmap I sent a video of is more fatiguing than either of these maps, simply because of the fact that you have to press so many buttons (about 2000 presses in a 4-and-a-half-minute period) so fast for so long.
Ok then lets talk about the pro scene, League of Legends is the biggest game currently. Again, completely different type of game. League requires teamwork above all else. Huge difference right off the bat. You'll never make it to the top without other good players teaming up with you to help you get there. Games last around 30-45 min on average. The game has a high learning curve due to the number of champions currently in play, I believe 115. Getting down to the gameplay and skills required, it is kind of a mix of DK and osu!. The beginning of the game is kind of like DK...you have to farm minions and last hit them at just the right time to get gold...kind of at the rate of jumping barrels. Toward the end, during team fights, it is more like osu! because you have to have quick reaction times and make fast decisions/keystrokes to defeat the other team and outplay them. These guys play all day every day. They're keeping their strategies on the cutting edge as the meta changes with each major tournament, champ release, or patch update to the game. Strategy and teamwork are required above all else to succeed at this game. Your goal is to outthink/outstrategize/outplay the other human team of players. As far as raw individual skill goes, I would say out of the 3, in my opinion this requires the least. It's all about teamwork.
I don't think your comparison to osu! is that great. I don't think osu! requires reflexes at all, nor do I think League of Legends requires fast keystrokes. League of Legends is about teamplay, knowledge, and decision making. Osu! has no decision making involved, and is instead only about speed, accuracy, and the ability to read/plan ahead.
So is it easier to 100% perfect every osu! screen, get 1.2M on Donkey Kong, or win the $1,000,000 League of Legends world championship? Probably impossible to say. Osu! definitely requires the fastest reflexes. Donkey Kong requires the most endurance. League of Legends probably requires the largest time commitment and teamwork. They're all fairly different skill sets.
Among these, it is safe to say that 1.2M on Donkey Kong is the easiest. There's already a player capable of it; the random factors of Donkey Kong seem to be the main limit to Dean's success in this. However, even if nobody was capable of it, I'd still say 1.2M on Donkey Kong is the easiest. Like I said, there are thousands of people playing League of Legends, practicing for 12 hours a day. You have to be
extremely dedicated and skilled to win the $1,000,000 world championship. As for 100% perfect on every osu! map: Assuming we exclude the ones which are impossible to get 100% even for a bot, and if we only count the more recent ranked/approved beatmaps, it'd still be ridiculously difficult and time-consuming just because of beatmaps like the two I linked up above, and all the crazy beatmaps like these:
http://osu.ppy.sh/p/beatmap?b=129891&m=0&nm=1#scoreshttp://osu.ppy.sh/s/41823http://osu.ppy.sh/b/264090http://osu.ppy.sh/b/172662http://osu.ppy.sh/b/133938http://osu.ppy.sh/b/226605http://osu.ppy.sh/b/83975Even if we don't take into account all the time players would have to spend grinding 100% scores on easy maps, and just pretend that the hardest of the hard are the only ones to consider, I still think it'd be an impossible task. Tell someone to 100% the fourth map I sent in that list and he already lost the challenge.
Also, osu! doesn't really require reflexes or reaction time. The lowest amount of time the circles ever give you to react is 300 ms. However, even this kind of reaction can be nullified with enough memory (though seeing circles flashing that fast in front of you really can mess with your eyes sometimes). Most of osu! is speed, rhythm, and planning ahead/reading.
Ultimately, my argument wasn't that one game is inherently more difficult than the other. However, osu! and League of Legends are
clearly more competitive than Donkey Kong.