Pikachu:
Chances of winning: 60:40 Pikachu's favor
What enemy thinks: (Have not discussed samus)
How does Samus do well/win
Samus will have to rely on her spacing game. When pikachu gets in, get him out as quickly as possible with up-b OOS and maybe some SH dair comboes. However, pikachu's projectile is quite good also, so be careful not to tack on useless %. Most pikachu's will be recklessly charging in, maybe with the aid of a t-jolt, since most people know what samus's camping game is made of. However, this leads to easy punishment with SH dairs, d-tilts, and up-b OOS
Pikachu's recovery has too many options for you to normally get a spike on the pikachu, so that method of killing is out the door. However, pikachu is still relatively light, so you should be killing him around 130% (Pikachu's Momentum cancels are really good) F-smash outranges everything pikachu has besides F-smash and D-tilt.
D-tilt is a great move against pikachu. It outranges every attack but F-smash, and that is too slow to come out. In this matchup, i think you should sacrifice d-tilt as a kill mvoe and use it as a damage racker and a spacing tool. Pikachu's will not expect it to come out so fast and have such a large range, it can be quite overwhelming.
Pikachus will try to use their small size to get around your projectile/zair use. You cannot SHMC a smash missile and hit the pikachu if they duck...it just doesn't work. Weak missiles and zair, however, can really keep pikachu away if they don't really know what is coming. This slow % adding can really hurt the pikachu since he is relatively light.
What to watch out for:
Pikachu has comboes, lots of comboes that can be improvised. However, through use of your shield and up-b OOS, you should cut down on the pikachu being too aggressive. However, if you eat a FF fair from the pikachu, get ready to be hit about 5-6 times with u-tilt, uair, more fairs, and nair (finisher).
Pikachu's game is mostly bait and punish, so the use of our T-jolts help all of our game. Not only can it tack on extra %, but it can make you shield (run in for a grab) or put you in enough hitstun to combo you.
Pikachu's killing potential is surprisingly amazing. All of these are kill moves, starting with the strongest: Thunder (Sweetspot hitbox, where it hits pikachu), F-smash (100% near the ledge), U-smash (120%. normally accompanied with a thunder to kill you off the top), Nair (130 near the ledge, 110% off the level), D-smash (140%. Easily DI'd out of if you SDI upwards), and u-tilt (160%. normally accompanied by a thunder) Some thunder setups with u-tilt and u-smash are extremely difficult to get out of. 75% of the thunder set ups WILL hit you at high %, assuming the pikachu doesn't mess up. You should be wary at high % of being near the pikachu, because he has a plethora of moves that pack a punch.
What stages samus should counterpick/ban
Pikachu is good on a lot of stages, he only has 2 bad ones. Those are brinstar, and luigi's mansion. Although i used to play samus, i never really knew good counterpicks...so if those 2 stages are good for you, CP them all day. Pikachu's best stages are Norfair, Delfino, Frigate, and RC (besides the neutrals). The best neutrals are smashville, FD, and Battlefield (for stage striking purposes).
Hopefully this helped. I never played a pikachu as samus so it was hard to write it from the samus's perspective.