Both her and Alison Brie as Annie are so good in it. On the whole, Britta is probably the funnier character, but Annie does a wider range of stuff because Brie is a more versatile actor. It’s close, IMO.
Not a coincidence that the funniest female sitcom characters are the ones that are not the token love interests and stuck in the will-they-won't-they storylines. Elaine Benes, Britta Perry, Sweet Dee, Liz Lemon, etc...
If I recall, Britta and Elaine started off as a potential love interest, but they abandoned that pretty quickly and they gradually became more interesting characters when they were part of the group's antics.
Dee from It's Always Sunny took that to another level by becoming the butt of the gang's jokes.