killing method: Death Note; anyone whoe name he writes down in this notebook, if he knows the person's name and face, will die regardless of means and range; a clear pick for the #1 spot.
killing method: Death Magic; Zeref's special magic simply robs whatever it touches from its life force, killing it instantly. Plants wither and trees dry out in his presence. He is also the supposed creator of numerous evil relics, such as the Lullaby, a flute that kills anyone who hears its melody.
killing method: A single punch. No, seriously, that's it, that's Saitama's power. If he really wanted to kill you, he could, he would, and he will do it in a single punch.
killing method: Geass; Lelouch's power is the Seal of Absolute Obedience, activated upon eye-contact. when looked into the eye while the Geass is active, he can issue any command that the target will obey without hesitation, even if that command is "Die", which results in the target committing suicide.
killing method: pressure point; Kenshiro is an OP character that can kill any pponent by simply touching them in a certain point on their body, resulting in a "pressure point effect" that kills them gruesomely soon after.
killing method: speaking; Eu's words can manifest in real life, so by saying "die", anyone that hears it dies instantly; that's why she rarely speaks and communicates via notepad.
killing method: Solver of All Formulas; the Mad God residing within him can analyze any substance, living beings included, and disintegrate them to oblivion; this even worked against supposed divine anti-god artifacts;
killing method: Imperial Arms Murasame; it's the sword Akame uses that can kill in a single hit; even if as much as a scratch is inflicted, the victim will die soon due to the cursed poison of the blade; it's so dangerous that tending the blade by Akame is life-threatening to herself.
killing method: vector manipulation. Not to be confused with Lucy's Vectors, Accelerator can control the vectoral direction of any object or flow, including the bloodstream. One touch with a human being, and he can reverse the flow of the bloodstream to instantly cause... well, use your imagination.
killing method: Candy Beam & Absorption; Boo can fire a beam that turns whatever it hits into whatever he wants, usually candy and other sorts of food, which he then eats up. He can also absorb enemies by surrounding them with his pink play-doh-like bodyparts and fusing with it.