That kind of depends. A Mercy doesn’t really have an effective weapon. Killing her is more like cutting off her avenues of flight. If you limit where she can go, her last hope is relying on her small hitbox. Her pistol doesn’t deal enough damage even with all headshots to be dangerous against anyone at full health.
Ana on the other hand has some of the strongest non ult abilities in the game. If you land a sleep on the enemy Rein, they have no shield for 8 seconds. That’s enough to get 2 picks, minimum. That alone holds the point. If you nail a DPS, their team damage drops through the floor. If you get a support, they lose a massive chunk of healing. Not having Trance available is often the difference between a team wipe or a successful repel/attack. Even if she dies, a Hail Mary anti is sometimes enough to kill someone or it could be just enough to let your Rein win the Rein-Off.
Moira’s abilities aren’t quite as game winning. Her healing spray is like Lucio on steroids. 80 HP per second to everyone in a group is insane. It makes it extremely difficult for flankers to kill their targets unless they get headshots. Her yellow ball is free healing. If you don’t have anyone who can heal a Pharah, she can literally just toss it into the air and walk away. Inside of buildings, her purple orb is basically slaughter. She doesn’t even have to survive to kill whoever was attacking her with it. Her fade is one of the best movement abilities in the game, which massively helps in keeping her alive. If you time it well, it can even dodge a point blank Dva bomb with no cover.
Both of them have impacts on the game that persist after they die. They aren’t as powerful as rezing a team mate but the difference is that neither of them have to risk being in a terrible position and being immobile for 300 years and even still having their ability negated.