Thomasina gets the first words in the play, and dominates (or at least holds her own) in every scene she's in. And more importantly, it's her development we follow – while the other characters of her century don't change much (or if they do it's in response to her), she learns lessons in life, love, and physics.
Thomasina may get the first words in Arcadia, but Hannah gets the last ones. And while it's Bernard's Byron question that drives the plot, Hannah's diligent resistance to his obnoxious stubbornness is more heroic. When she takes down Bernard, she's the one we're rooting for.