Megaman x

X can actually do a hyduken! First of all, you have to have gotten everything and have full power on everything (including the special blaster you get before zero dies).
If you are having trouble, just use this password :
7431-3842-8523

Now, go to the Armored Armidillo's stage and get to the end without loosing any life or weapond powerups (you can loose them, but make sure you refill them before the end of the stage). Before going through the boss doors, teleport out.
Do this 4 times. On your fifth time, climb the ledge and you will see an upgrade pod that will give it to you.

You will need full power to use it and it is done the same way as Ryu does it. It is so strong that it can kill anything (except the third form of sigma) in one hit!