Seeing how Pikachu is now the only character with an even matchup against Meta Knight, we felt like we should explain this. There will be two parts to this explanation: the matchup itself and then a comparison with other contenders for an even matchup against MK.
Pikachu vs MK is basically all RPS situations. MK has the slight edge on spacing and safety, but Pikachu gets much more rewards per hit because of chaingrabs and followups. Pikachu's projectile also helps, because it makes MK initiate. Couple this with the fact that Pikachu is an amazing punisher and you get a pretty good situation for Pikachu. MK's safety is his main attribute in this MU, and it is mostly for pokes more than camping. Pikachu's uair is a really good move to combat dair camping, and planking is rendered semi-useless with T-jolts. MK's safety translates to his kill game. D-smash is pretty safe and at max range it is near impossible to punish without a power shield. However, it won't kill consistently until the 130% range while Pikachu's more punishable kill moves will take a stock closer to 105-110% (F-smash or u-tilt/u-smash thunder).
Now, there are 2-3 other candidates that could be even with MK, and those are Diddy, Falco, and Fox. A huge benefit that Pikachu has over them is the fact that he doesn't get gimped. All of the other three are in REALLY bad situations if they are ever off-stage, while Pikachu is pretty safe. Diddy has better ground control than all, and Fox/Falco can camp better than Diddy/Pikachu. Pikachu has similar abilities, but is outshined. The gimping is a huge factor because there is honestly more of a chance of Pikachu getting a fluke kill due to MK having bad DI than getting gimped. Pikachu also doesn't suffer from stages like the other three, since he can definitely handle RC, Delfino, and Frigate (although Pikachu players will always ban Brinstar).