It's probably impossible to compare a game like Donkey Kong to a game like osu!. They're pretty different.
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.
Donkey Kong is also tough, but from the outside doesn't look hard at all. An osu! player would probably look at Donkey Kong and say it looks boring. It's a different kinda tough...more endurance tough. It's also random, so you can't just memorize the game and then execute it...it requires on-the-fly thinking. Barrels that won't steer, never knowing when a wild barrel will be thrown, multiple long springs in a row...etc.
I am swayed toward the DK side, so take this with a grain of extra fine salt, but would a way to compare the 2 go something like this maybe...osu! requires to you move the mouse around the screen and hit keys based on a pre-determined pattern. Donkey Kong requires you to jump over barrels and avoid obsticles a couple thousand times during a full game. Would it be fair to say there are about 1000 keystrokes that must be executed precisely in an average expert osu! run? DK has around 2000 (rough estimate) jumps that must be executed throughout a full game, plus random elements like wild barrels, unsteerable barrels, etc. Toss in the spring screens, and amount of time it takes to complete a full game (2.5 hours +). I don't know, they're pretty different. Probably for some people osu! would be easier, and others DK would be easier.
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? Would fatigue become a factor? (again I'm not picking on osu! here, just making friendly discussion : )
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.
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.
It'll never happen, but I would love to see someone from TSM or Curse Gaming try to get 1,000,000 on Donkey Kong. I bet some if not all of them could. Who knows how long it would take them. I'm sure osu! players could get 1M on Donkey Kong, just like I bet some Donkey Kong players could near perfect some of those crazy osu! screens.
The only way to fairly judge people's skill levels in osu! or Donkey Kong would be to start from scratch, keep track of the number of games it takes to get a certain score, without help from anyone else or any outside source, then compare the number of games it took each person...the lower number of tries would have to be the winner.
Oh and if any of you want to join me and go for the $1M on LoL, we could beat those guys. Maybe.