I've loved the idea of Sonic even before Brawl. Electronic Gaming Monthly made an April Fool's Joke one year saying he was in Melee, if you completed a terribly tough task. I suppose it was inevitable that Sega's greatest warrior would show up. I've always liked him, he was good in both Brawl and Smash 4.
Just like the previous games, Sonic is by far the fastest character. He makes Captain Falcon look like a turtle in a race. He's also got a crazy high jump to him as well. He's very agile and on large stages, he's nearly impossible to hit without some skill or some items.
His special moves kind of all revolve around his spin dash attacks. The regular homing one is very good, and it can annoy an opponent. It hits more than it misses.
His Final Smash is (duh) Super Sonic. Where he transforms into a golden form with the help of the chaos emeralds. It's not that powerful of an attack but you basically fly in a rapid motion across the screen. You can control it and easily get at least one KO, most of the time.
When Kirby takes Sonic's moves, he gets the spin dash homing attack. Which is a decent move to Kirby's moveset. Almost as cool as his cute new Sonic hat/wig.
While I don't main Sonic in Ultimate, I did enjoy using him online from time to time. He does really well in 3 player battle royales in stock matches. I usually could easily dodge the other two fighters, and I'd let them wear each other out and I'd get the easy win. Though with one on one battles, he's not as good. For his speed and agility, he lacks some power to him. He's certainly stronger than Sheik, as most of his moves are worth doing.