Armada has said it a lot, it's mostly reads and knowledge (punish/combos) that win most encounters on and off stage.
There are exceptions for some characters that rely heavily on reaction like Sheik and CF, but even than it's choosing the best option coverage (and limiting options) instead actually reacting to 1 option out of many.
After playing this game competitively for a while I can tell you one thing for sure: A large majority of players overestimate their reaction time to a massive degree. Most believe they that see things, react, and input within 12-15 frames but really it's far worse than that (like 30-40 frames) given that they are truly reacting to a variety of options. Add in nerves, adrenaline, input lag, display lag, etc and it only gets slower.
There's a ton of high level evidence of this and it's why simple tactics like tomahawk grabs or Samus WD back U-tilt still work. Edge guarding is where you can really see how slow reaction time is for humans, quite often top level players will fail to react to things that are telegraphed a full 1-2 seconds in advance. There are a lot of possibilities defending a ledge and recovering to ledge, even for pros who have seen every scenario 1000s of times it's difficult to react.
Personally I believe someone with crisp, fast, and tight movement will always beat anyone that has above average reactions but mediocre movement. Melee is far too fast, it favors aggressive play and MU knowledge.