Most appearances for a playable third-party character in Super Smash Bros

Sonic first burst onto the Smash Bros. scene on 31 January 2008 in Super Smash Bros. Brawl (Nintendo). Sega’s iconic hedgehog has been a stalwart of every Smash Bros. roster since, featuring in Super Smash Bros. for Nintendo 3DS and Wii U (2014) and in Super Smash Bros. Ultimate for the Switch (2018). Konami’s Solid Snake, who made his debut alongside Sonic in Brawl, skipped Super Smash Bros. for Nintendo 3DS and Wii U, leaving Sonic in the clear as the having the most appearances for a third-party character in the series’ history. Sonic is an A-tier fighter and, as you might expect, has the fastest dashing speed in the game.

This counts as 'appearances as a playable character', as opposed to assist trophies and other non-playable forms.