Sonic eh? I'll bring him up
I've played quite a few Sonics, and they're pretty mean. However he sufferers from approaches. His Spin Dashs just beg to be Pivot Grabbed, and he fairs no better just straight Dashing or Grabbing. Unless the Sonic Player has good reflexes, he's going straight into your mouth. I find that the Sonics I've face realize that Fastest, isn't fast enough. I guess experience that those approaches will be useless against Yoshi, so they will attack from the Air
Anyways, I find his Airs quite good. Nair is a waste, and Dair is easily punished, but Fair, Uair, and Bair are quite nasty. UpB works pretty well as an attack (That Spring is scary O_o), but the remaining Specials are pretty pointless against Yoshi. Homing is just too easy to see coming, and Side/Down B can be countered easily. He can do that jump out of it thing to intercept you in the air. His ground game... is actually quite lacking His tilts are pretty slow, and same with his smashes. Sonic suffers a huge problem in killing, but can punish really easily if you leave yourself open, what with his trademark speed and all. Edgeguarding Sonic is quite easy because he can only go one direction with the Spring, but he might SideB back to the stage also though. I say it's 6:4 Yoshi, but I want to see what Burnt has to say about him also.
I also want to bring up Toon Link, and Link.
Toon Link is pretty tough, but he doesn't have much of an advantage. He has a decent Ground game with good range, but not good enough. Yoshi can still get through his attacks quite easily. His Airgame is decent, but Yoshi's Bair can get through them. They're still pretty deadly, so look out for them. His Dair, can be Umbrellaed with an Straight up UpB Egg, and if he misses, he can be punished badly! His ground game is the same deal. Pretty deadly, but suffers from the same Range Problem, which Yoshi can still break though.
His projectile game is decent. His Boomerang is quite good, and same with his Bombs, and he will use them. Bow isn't really that great for spamming due to the slowness of the arrows. Edgeguarding Toon Link Physically is not really worth it, just use your eggs. You can interecpt him if he UpB's and doesn't go for the edge. Toon Link can Edgeguard good as well with his own Projectiles, so look out. I say this matchup is 4:6 in Toon Link's Favour.
Now Link. He's nasty to Yoshi! He plays the same as Toon Link, but fixs the range problem, which is a huge threat to Yoshi! Yoshi will have a much harder time approaching in both the Air and the Ground. He has faster and better projectiles which make it ever worse! Otherwise, hes about the same as his Younger Self. Only other differences is that he's Slower (But can do a Sliding Usmash, like Snake, but worse!), Can be Chaingrabbed (But just like Ike, he's tricky to grab), and can't recover very well. I say 3:7 to Link. No matter what you put Toon Link, Link will always be a Point ahead.