The Most Overpowered Piece

Yeah, the queen and knight is a powerful combo, but the knight can actually checkmate all alone without the queen! It just needs the enemy things surrounding the king for this to work. This checkmate is called "smothered mate" and this game from an old video of mine is one example of it:

There is a huge difference between being able to force a checkmate, or the ability to set up a checkmate position with the aid of the opponent ('help mate'). The point of the O.P. is that on an empty board the QN compound ('Amazon') can force a King with checks to the edge, and checkmate it there.

It is not the strongest piece that has been used in major chess variants, though. The Lion in Chu Shogi can make up to two King moves per turn, even if the first of these is a capture, and is worth more than the Amazon even on a 12x12 board. The strongest piece I have ever encountered is the Fire Demon of Tenjiku Shogi. It is so strong because it not only captures the piece on the square it goes to, but also annihilates all enemy pieces standing next to it