B button

From SmashWiki, the Super Smash Bros. wiki

The B button is the small, red button on the GameCube controller, the large, green button on the Nintendo 64 controller, the rectangular trigger on the back of the Wii Remote, or the small, clear button towards the bottom right of the Classic Controller. It is used to activate special moves in Super Smash Bros. and Super Smash Bros Melee. In Super Smash Bros. Brawl, controls and control settings can be changed. By default though, when using the Nunchuk, Classic, or GameCube control schemes, the B button activates special moves as well. With the Wii Remote alone though, the 1 Button acts as its B button, performing special moves, whereas the actual B button is used to shield your character.