So, in the MCU movies, Peggy Carter is, for the most part, Steve's love interest. She's shown to be more than 'the girl' in The First Avenger, but in that movie her sole purpose is to be Steve's love interest. She's an old cameo in The Winter Soldier, and gets reasonably fridged in Civil War. Her only role in Endgame was to be Steve's love interest in a five second cameo. The best of Peggy Carter in the MCU movies is her two minute cameo at the start of Ant-Man. Fucking Ant-Man is the best we've seen Peggy Carter in the movies, where she's more than a love interest. But that's it, ***in the movies***, Ant-Man is her best appearance.

In the rest of the canonical MCU (not counting any non-canon multiverse things), Peggy Carter is wonderful. Firstly, Agent Carter (the one-shot), shows Peggy at the SRR proving that she's more than the dead Captain America's girlfriend (and kicking ass). Then in her two appearances in Agents Of S.H.I.E.L.D. (a total of about five minutes over two episodes), she's rounding up Hydra's loose ends and the Red Skull's devotees, literally leading the Howling Commandos, and she's also looked up to quite a bit by my personal favourite AoS character, Jemma Simmons.

The best of Peggy Carter in the MCU is in her own show, Agent Carter (surely, the best of a character would be in their solo project, but that's not the point I'm making) In Agent Carter's short lived run, we see Peggy in and of herself, even within the realm of Captain America, but she's not defined as Captain America's love interest, and rarely is besides a passing comment or two.

In Conclusion, Peggy is just a love interest in the films, but in the Agent Carter short and show, Agents Of S.H.I.E.L.D., and, surprisingly, Ant-Man, she's an actual character who exists distinct from a love interest. I also find this really interesting, because the people who wrote Agent Carter, the best of Peggy Carter, also wrote the Captain America films, and the two most recent Avengers films, the latter of the latter, alongside the former, is among the worst of Peggy Carter.

Thank you for reading my TED talk.
My opinion on Peggy Carter in the MCU Movies