GameCube Controller (Original Wii RVL-001 only): Works only in GameCube backward compatibility mode (playing GC discs). Does not navigate Wii Menu or play Wii games.
Classic Controller / Classic Controller Pro: Plugs into Wii Remote bottom. Still requires Wii Remote to function.
USB Keyboard: Works only for text entry (Shop Channel, Internet Channel, some games like Animal Crossing: City Folk). Cannot navigate menus or play games.
Homebrew (CFW): Some homebrew apps (WiiMC, emulators) support GC Controller / USB HID via custom IOS — but you still need a Wii Remote to launch the Homebrew Channel.
Bottom line: A working Wii Remote (+ Nunchuk for many games) is mandatory. Buy a used official Nintendo Wii Remote (~$15–$25) — third-party remotes often have poor IR tracking / build quality.