I'll mention the Yoshi

matchups that I have the most experience with:

Bowser: -1
Yoshi is absolutely not +3 in this matchup, it's even at best. Yoshi has no recovery versus bowser, and bowser has plenty of options to get Yoshi offstage and large hitboxes to cover the few recovery mixups Yoshi has. Bowser, of course, also has the potential to break armor much earlier than most the rest of the cast. Although Yoshi does win the neutral, its not overwhelming to bowser like the fast characters and Yoshi does not have a reliable grab or projectile.

Charizard: +1
Yoshi can get out of most of what charizard wants to do, including his bread-and-butter up b kill off the top which Yoshi can armor through and retaliate with a down b kill whenever charizard attempts it. Yoshi lives though upthrow to pretty high percents, and charizard's are not reliable enough to knock yoshi out of jump armor unless you are a god with your tipper placement. Yoshi has a hard time fighting charizard on stage in the neutral, but he has huge combos and still lives pretty well against charizard despite his recovery nerfs.

Diddy: +1
Maybe I'm just good at this matchup because a lot of other Yoshi players and Diddy players seem to disagree with me.

Donkey Kong: -1
Yoshi v DK is kinda like Yoshi v Bowser, except DK has huge combos onstage, but has a tiny bit harder time edgegaurding yoshi. I play a lot with Noghrilla (whos already given his list in this thread) and mojohnbo, two of the last remaining good dk players.

Falco: -2
Free win if they come from melee and don't know what your character does. Free loss if they have any experience against Yoshi. All of Yoshi's aerials lose to Falco's. Yoshi has a hard time dealing with lasers, especially with a glitchy shield. Yoshi can't deal with shield pressure and has a slow roll (and also a glitched roll out of shield) and no shield grab. Yoshi can't grab falco reliably with the grab nerfs, but he needs to to get his huge combos started. Also falco's shine breaks armor at 0%. This matchup mostly comes down to trying to armor through something of falco's, hitting a nair, and either getting a techchase into grab or an edgegaurd off of it.

Ganondorf: -2
Basically a better bowser.

Ice Climbers: +2, maybe +3
Yoshi's egglay wins this matchup. Long range egglay to separate them, DJC nairplane nana offstage, popo is ez$$. alternatively, dtilt and dsmash destroy the climbers. Yoshi has the platform tricksies to not get grabbed, and even if he does get grabbed, so what?

Marth: 0
Yoshi got nerfed in some of the most relevant ways to marth: grabs and recovery. This matchup was certainly in Yoshi's favor in 3.5 but I think it's even now. There are valid arguments to make for this matchup being slightly in Yoshi's favor, but dont let anyone fool you into thinking this matchup is +2 or better for Yoshi.

Meta Knight: 0
I wrote a lot on this matchup in the MK thread. Yoshi used to be MK's worst matchup, now its even.

Peach: -3
Yoshi's second worst matchup. Yoshi can't challenge peach in the air or on the ground. Yoshi has no edgegaurds and no combo game versus peach. Yoshi has no way to deal with float or crouch cancelling (esp with the downair nerf). All yoshi can do is throw eggs, which are pretty slow. Thankfully Turnips are also pretty slow, and thats what makes this matchup not quite as bad as Zelda.

Roy: +1
Really good matchup before, not affected too bad by the Yoshi nerfs. Roy's typically stay on the ground a lot, which allows Yoshi to get easier grabs on roy than on other characters, and Yoshi has really good conversions from grab on Roy.

Toon Link: -2
Toon link has always been a hard matchup. Toon Link just way out neutral's Yoshi with projectiles, especially bombs.

Wario: 0
Yoshi's nair stuffs out Wario's side b, and Yoshi can get out of a lot of wario's stuff. Yoshi also has pretty good combos versus Wario. Yoshi can reliably challenge Wario in the air.

Wolf: +1
The easiest of the spacies since Wolf can't camp Yoshi out with lasers as effectively and Yoshi can CC the Shine. Wolf gets combod and edgegaurded way harder than Yoshi, though he does win neutral.

Zelda: -3
By far Yoshi's worst matchup. Yoshi has no way in, no combos, no edgegaurds, and cannot throw eggs because Zelda can reflect them or use her side b. Side b traps mean Yoshi can't approach from the ground and all of zelda's multi hit moves and her giant hitboxes means yoshi can't approach from the air (even with DJ armor). And like I said for peach, Yoshi lost his best anti CC tool in downair, which is now punishable on hit.