Alright, well I decided to try to start a Pikachu match up so here it goes
Pikachu's strongest moves are his Fsmash (fully charged=28%) Side-B (full=25%) Dsmash (full 21%)
He is a surprisingly strong character for such a small and cute appearance. He can be pretty dangerous on the field of battle (especially for Sonic) Out of all the match-ups, I believe Pikachu can be one of the hardest and he definitely has the most advantages so if you're forced to play against one with sonic, try using this advice.
Pikachu players are not afraid to spam a thunder bolt [B special (tbolt)] or their thunder (Down-B) So it's hard to get close to Pikachu, and it's hard to stay away from him without getting hit by a tbolt. These can also cancel both of your spin dashes, pretty much meaning you can't use them, and THAT'S NO GOOD. He also has better ground game than Sonic so here's what you have to do. Sonics airiels > Pikachu's airiels. Try to keep him in the air. When trying to approach Pika, the first thing he'll want to do is tbolt. If this occurs, sonic's fsmash, ftilt, and dash-attack can cancel one of his tbolts. Also try shorthopping+airdodge over them, or just plain dodge roll. (Also, even though they can be canceled, a spindash into a jump + dair can work quite nicely in an approach attack situation.) Once you have approached him, GRAB AND THROW HIM UPWARDS. Grabs work great. BE CAREFUL THOUGH. When starting a grab, Pika may try to perform a thunder, if you're fast enough, it won't work, but if you're off by the slightest second, the animation will stop, but the thunder will still strike, breaking the grab. Thunder is a real b*tch because the damage may not be that high (10-17%) but it's a freakin long attack, hard to stop, and has great priority and range. Once in the air, start to uair. Try to keep juggling him as long as you can! Make sure to mix in Fairs (high priority to pika) and bair are good for knocking away) spring+uair can work as a great kill too, but as always, beware the thunder. When Pikachu is off the stage, he has two options for recovery.
1) SideB (skullbash)
2) UpB (Which can be used twice in different directions)
The best way to kill is by simply bairing or fairing pikachu too far off stage for return. Pikachu is light weight so this can be easily performed once he's at like 75-90% damage. Another way to kill is by gimping. Hey, if pikachu isn't ashamed of spamming, why should you be ashamed for gimping? (some think it's a cheap kill) anyhow, you're not going to get too many of your kills this way, but it's great if pikachu is directly vertical and underneath the stage edge. All he can do is upB (will prob either shoot straight up, or go diagonally left first, then diagonally right to grab ledge) By edgehogging this option is gone and will lead to his imminent death. Also bair or fair spiking pika into the side of the stage works very well!
Now say the tables are turned and its YOU trying to recover. Although sonic has amazing recovery and is pretty much impossible to gimp, pika will stand at the edge and ***** his thunder. When using upB, get the timing correctly so you can pass right through the thunder with an airdodge
Now I'm kinda skipping around a bit, but let's chat about the ground game. Pika's ground game pretty much owns Sonic's. His dair almost has kind of a vaccum effect that if you're too close you get stuck in the center of an electrical spin. Fsmash has starting lag, but has wide range and great power. An approach for either of these moves are tilts. ftilt works wonders. Spacing is the key element in the ground game. There is one spot that beats pika's ground smashes and thats directly infront of his fsmash. When those little cheeks of his start charging electricity, dodge roll BACKWARDS. You will be just out of reach for this move, and a quick fsmash (recommend cstick) will get a good hit. This also works well on thunder(be more cautious with thunder though. that move is hard to analyze seeing how it lasts so long and pretty much protects him from all angles) but there is a point where it's safe to hit him (hard to explain but you will know when fighting)
When in close combat, a dodge in place+ smash/tilt works well on tilts or quick smashes performed by pika. Even a good neutral A+A+A combo is effective, the only problem is Pika's neutral A which is almost as bad as being hit with a fan so don't get caught in that
This will probably be a long, tough battle, but it's not impossible to win! It can be done if you play smart.
Well I think I said all that I could say...I hope this helps, and please fix any errors I made or add anything to it. I think that this can be a good start though.
Or maybe spring...