Pit is actually one of the "dedede counters" characters. His arrows are faster than Waddle dee toss in terms of being spammable, countering the Waddle Dees. Getting him to face you in close combat is your best bet.
Against Pikachu, use Waddle Dees to block Pika's Thunder Jolts (Neutral-B). If he isnt aware, I'd use 2 waddle Dees (one might be waddle doo) then charge up the Jet Hammer. While the Waddle Dees Hold off Pika, close in to him and Unleash the Jet Hammer.
Diddy Kong is another one of the "Dedede Counters" Characters, with the annoying banana peels which will limit your movement options. Not even my Special Jet Hammer strat dont work. I dont think you can Chaingrab this guy
Vs. Metaknight, Be patient and wait for him to come to you, since he lacks projectiles. But I dont know anything else.
Sonic, well I cant tell you how to beat him since he gots too much speed and his annoying spring. His moves are bizarre and cant tell which one he gonna use. I Suppose waiting for him to come to you is your only option. Dont think you can chaingrab him either.
When I'm using Dedede, Im good against Slow characters, 50/50 vs Pikachu, and 50/50 vs Ike. I use Bowser, Pit, Zelda, Wario, and Metaknight to counter the others.