In the Beatrice-Benedick plot, Beatrice is arguably more important than her lover, not only because she’s sharper than he is, but also because she holds quite a few of the cards. Once they fall in love, Benedick is always the one making advances, and Beatrice has the option of accepting him or rejecting him. Benedick is the first to admit he’s in love, laying himself bare, and Beatrice controls the action by choosing to return his love. Similarly, in the last scene, Benedick calls Beatrice forward, and she comes close to rejecting his love publicly.
Beatrice has a handle on the action, the way protagonists should, but she also is the character we get to know most deeply. This arguably makes her good protagonist material. We see several different sides of Beatrice – her sharp and prickly wit, but also her vulnerability. We know she’s apprehensive about love because she drops hints about being hurt in the past by Benedick. Because of her vulnerability, we know that the decision to love Benedick is a really big deal for her. Beatrice’s choice also means a lot because it shows she’s giving up a good deal of her personal freedom. While marriage would be a taming of both wild characters, the play closes with a joke that Benedick always has the outlet of cheating. Beatrice, by contrast, is really giving up her life to Benedick. She’s a good woman, and a good person, so it’s likely she won’t cheat; her acceptance of marriage is more significant than Benedick’s because she has more to give up.
Some might argue that Claudio is a protagonist within his plot (the Hero-Claudio love story) because he definitely has the upper hand over Hero. He’s the one who admits his love for the girl, and her job is simply to accept him (or whoever her father chooses). Claudio is definitely the power player in their relationship and much of the play; his feelings are forceful and determine much of the play’s action. Claudio rejects Hero, and causes the whole hubbub that follows. However, we don’t consider wielding power to necessarily be a quality of a protagonist. He’s definitely not the most compelling character, and he never seems to learn anything or grow throughout the story. At least in our point of view, Claudio is not a protagonist.