Thundersn0wHardest game is Melee
Just to elaborate on why melee isn't hardest and Quake and SC are probably the hardest:
SC takes tons, if not, probably the most in terms of multitasking in a competitive game. It requires:
High knowledge(there is A LOT to know), High responses, High APM(or speed in general of playing), multitasking of macro and micro to the core, and mistakes can become very punishing.
I'm not saying melee doesn't have a lot of those, but I'd definitely say it doesn't require as much multitasking. In the end it becomes "choose a field, two players choose a character, fight". I can understand that you have to somewhat have the field memorize and know certain spots to know about, while also knowing about the enemy character in general and aspects of it. But most players at the highest level already know about that and have it embedded by the time the match starts, so what's left is actually playing the match, movement, predicting your opponent, techs, DI's, and etc. But the thing is, is that the players are pretty much able to completely focus in that one battle between two characters, and don't have much else to worry about.
Quake on the other hand may have you think "Well, Quake is just the same thing but in FPS!". Quake is a lot different from a 2D-like fighter game. Quake requires aim(important), movement(in a different way from melee), knowledge(this knowledge is different), predictions, hearing(hear item pickups/weapon sounds/movement), and one of the most important: item timings. Item timings are absolutely important to keep an advantage in the game and ensuring you are able to withstand enemy damage. Quake has a need for aim(which is somewhat hard already at high levels), hearing, and item timings, and just those 2 last things somewhat add more to the knowledge part of quake that makes the game harder.
Don't misunderstand me though, I would definitely agree that Melee and maybe SF are the most skilled and hardest fighting games, but in terms of games overall? I wouldn't say it's the hardest. If I'm missing anything about melee that should be mentioned, feel free to post as I don't delve a lot into melee. Also I tried using some main aspects of each game to explain this rather than specifics(this post would be even longer). I still respect melee as a game being up there though.