Garchomp◊. Oh God. One of three or four "normal" gamebreaking Pokémon not intended as such, it caused a great deal of controversy in 2008 when people finally unlocked its full potential and it started annihilating teams left and right. There's a great many reasons as to why it's considered overpowered, excellent stat placement, great typing and Yache Berry among the most recognized. It got promoted to ubers when Platinum was released, and remained there for the rest of the 4th generation.
Metal Blade◊ from Mega Man 2. What the image says is true, but on top of that, the weapon can be aimed in multiple directions and can be fired many, many times from a full energy meter, while still being at least as damaging as the standard Mega Buster.