Which Super Smash Bros. game introduced the Final Smash mechanic?

Super Smash Bros. Brawl introduced the Final Smash mechanic.

Final Smashes are powerful, character-specific attacks that became a defining part of Brawl’s battles. A fighter normally activates one by breaking a floating Smash Ball, an item marked with the Super Smash Bros. logo. After breaking it, the fighter briefly enters a powered-up state and can unleash the attack.

The move differs dramatically between characters. Mario fires a huge rainbow-colored beam, Kirby transforms into Cook Kirby, and Fox, Falco, and Wolf use team-based attacks. Some Final Smashes deal massive damage directly, while others temporarily transform the character or trap opponents in a cinematic sequence.

Final Smashes returned in Super Smash Bros. for Nintendo 3DS and Wii U and Super Smash Bros. Ultimate, where players can also enable a Final Smash Meter that charges during combat. They are sometimes confused with ordinary “smash attacks,” but those are standard directional attacks available throughout the series; Final Smashes are a separate Brawl-era mechanic tied to special activation rules.