I never said these characters were top tier, but were among the best in the game. Diddy Kong cannot edgeguard as effectively as the big four can, but can rack up some serious combos super quickly. Sonic really doesn't match up well against any of the big four except, possibly Rosalina, and that's only when customs are involved; however, Sonic's focus is mainly a combination of trickery, bluffs, and in-your-face-style of fighting, and as such has some incredibly diverse ways in which you can play him. His unpredictability (especially since his custom specials can definitely take some people by surprise), and he even possesses some edge guarding capabilities.
Of all the characters in the game, a good Sonic player would have the best poker face in the game essentially. But it's much more than that, because good Sonic players make the opponents figure out how they play, and change up their tactics on the fly to keep the opponent guessing. Granted, this applies to everybody in some way, but Sonic does it better than just about anybody. Characters like the big four and Diddy Kong have their primary functions in place, but Sonic can do just about everything except camp. It helps that it isn't so easy to tell apart his side or down specials from one another based on face value, though as for customs, it's usually best to save those to take your opponent by surprise since he is generally at his best with his default moveset.
However, Sonic is at a disadvantage against the big four except for Rosalina (and maybe Sheik). Greninja can neutralize Sonic's air game and push him off the stage with Hydro Pump. Zero Suit Samus, meanwhile, can get away easily in the air and can set up combos with her paralyzer; Sheik might be an even matchup, but even her custom specials can neutralize Sonic better than most characters in general. Rosalina can stop a default Sonic, though custom moves can essentially turn the tide for Sonic and at the very least, even the odds.
Outside of that, the only blatantly poor matchup I can think of for Sonic is Little Mac, and that's because Little Mac has heavy armor in his neutral, and super armor in so many of his others; Sonic, meanwhile, cannot help but get close to him in order to attack. Characters with some sort of counter can also potentially stop Sonic, though that may require some ridiculous timing, in which case, only Lucario and Shulk have those sorts of benefits against him (and even in the latter's case, he can't spam it, either; it gets harder to pull off with each successive use).
Overall, if I were to make a top ten list of best characters right now, it would probably look something like this:
1.
2.
3.
4.
5.
6.
7.
(everybody ranked above him can kick his rear, though he has favorable matchups against most others anyways)
8.
9.
10.
Honorable Mentions:
Again, only the top four would be Top Tier in my eyes; everyone else here is looking to be the most viable in the competitive scene with perhaps five or so others thrown in for good measure.
