Laser Trident from Mega Man 9 is sweet.
Chain Rod (Mega Man Zero 2) is awesomeness incarnate. While using it I occasionally get the urge to yell "GET OVER HERE!" It's also awesome because you can hit shielded enemies in the back while they're facing you (jump, Chain Rod so that it just goes over the enemy's head, then the Rod will hit them in the back when it retracts.) On Hard mode, it's like a buster you can aim in 8 directions, at the expense of not being able to move while firing (unless you're in midair), and having a shorter range.
Magnet Mine (Mega Man X2) is badass. Guided, powerful, but slow so it's not overpowered.
Shock and Awe said:
FEAR THE METAL BLADES WITH THEIR 8 FIRING DIRECTIONS AND HUGE SUPPLY OF POWER!
I see your Metal Blades and raise you an Elec Beam. Storm Tornado is up there in the overpowered department too. Anything it touches that isn't a Metool (Hard hat dude) or a boss, dies.