What is the most effective method for the roundhouse kick?
You've got a one-adjective criteria there, and a vague one: effective.
Overall effectiveness might reasonably be defined as what helps you win reliably, or perhaps you'd prefer something less reliable if it meant the average or median injury you sustain is less even though the worst case outcome's less favourable. That kind of analysis depends on your opponent(s) as much as you, because the quality of their defense, conditioning, and counter-attacks determine whether you'll be able to use your roundhouse effectively. An incredibly powerful but overly telegraphed roundhouse might be useless against a very agile opponent, while a fast flicky kick might do insufficient damage against most targets on a larger, stronger opponent. It does depends on the target though - a flicky but "clean" kick to the jaw should still be a knockout. Trying to take someone out through thigh kicks, or breaking down a guard, needs more power than a kick to the floating rib or liver but will likely be less effective anyway.
Effectiveness of a kick is also a result of the unpredictability of the timing, catching the opponent moving the wrong way, coming in at an unexpected angle e.g. a very few people have the flexibility to lift the knee higher mid-kick and kick downwards into the neck, another variation needing less flexibility is to drop the knee while turning the torso away from the opponent and swinging the lower leg down into the opponent's face.
So - lots of considerations for any meaningful measure of effectiveness....