I'd say Sheik and Zero Suit Samus. These two are Zelda and Samus. I'd rather have Zelda and Samus as viable characters. It makes sense for Sheik to be better than Zelda, but it doesn't make sense for Zero Suit Samus to be better than Samus.
"My Power Suit is my greatest weapon. Designed by the Chozo, it boosts all of my natural athletic abilities while providing an array of defensive and offensive attributes. Boasting full armor capable of sustaining ballistic damage, life-support systems, my upgradable Arm Cannon, and Morph Ball capabilities, it truly is a part of me." (Metroid: Other M Art Folio, Power Suit, emphasis mine)
Why is it that if the power suit boosts Samus' natural athletic abilities, that Zero Suit Samus jumps higher and runs faster?
"I prefer to use my Arm Cannon, as it allows me to take out most threats from a safe distance. When circumstances call for it, however, I'm happy to engage the enemy hand-to-hand. The augmented power of my suit lends me extra power in combat-sometimes a well-placed kick is the only way to deal with a grappling Zebesian." (ibid., emphasis mine)
Why is it that Samus' jab lacks hit stun, but you can be certain you'll land all three jabs with Zero Suit Samus? So much for happily engaging in CQC! So much for that augmented power of the power suit! How about we get rid of Zero Suit Samus and give all of her buffs to Samus?
Going back to Sheik, why even have Sheik? She made only one canon appearance and we have seen Imp in more Zelda games. I'd rather have Impa, who is a Sheikah herself. Get rid of Sheik and change up Zelda's abilities so that she's not ****ty. Sure, she gives me trouble, but I'd rather have the original characters have better options. Furthermore, introducing Impa and introducing another Metroid character would not only reduce redundancies, it'd make more characters available without having to increase the large size of the roster.