-The Metal Blade from Mega Man 2. It can be thrown in 8 directions, is larger than a single Mega Buster shot, uses little to no ammo, and pretty much owns mostly every enemy in the game (asides from Springers, Sniper Armors, Quick Man, the Boobeam Trap, and the Alien). It also wrecks Bubble Man, Wood Man, and Flash Man, and in the 5th fortress stage, it can destroy Metal Man in 2 hits. A Robot Master is weak to his own weapon.
-Also Mega Man X's Hadouken, but they make it hard to get; pretty much you need to get every upgrade in the game, then fall down the pit before Armored Armadillo's boss shutter. Eventually, you will see a Light Capsule, and entering it gives you the Hadouken. It 1-hits every boss except Sigma's final form. While Metal Blade is less powerful than Hadouken, it's MUCH easier to get, as Metal Man's stage is one of the easiest in Mega Man 2.