During its Nintendo Direct today, Nintendo confirmed that the newly announced Super Smash Bros. Ultimate will support Nintendo GameCube controllers.
No specific details about wired versus wireless controller compatibility were revealed, but Nintendo did show an image featuring the Smash-branded GameCube controller released alongside Super Smash Bros. for Wii U.
The presentation also confirmed that all amiibo from the Smash Bros. series will be compatible with Super Smash Bros. Ultimate, and that player data from Smash Bros. for Wii U and 3DS you’ve saved to amiibo will carry over. In addition, amiibo of characters in Smash from other amiibo series will also be compatible with Ultimate.
Source: IGN.com Smash Bros. Ultimate Will Suport GameCube Controllers