i personally said melee because it combines being incredibly proficient technically (actually doing the tech, being able to combo, etc) but also a really difficult mental game (reads on DI, tech, rolls, etc, knowing what your opponent's going to do, and not tilting all throughout this, not to mention the crowd pressure).
but kev has a point, most esports are esports because they have incredibly high skill ceilings and therefore are hard as hell to play at top levels, you could make a case for each of them being the hardest except for maybe hearthstone