What's the need to choose one? They're all incredible in their own ways.
Arwen: Her brave decision, to choose mortality over the comfortable life of Valinor, family and all her kins. Knowing she'll end her life in a bitter way, she chose to give up all of it for her man. Who can't like her? It's not just her beauty, but her character that makes her a powerful character despite not appearing much in the books.
Lady Galadriel: I'd go for her for rejecting the temptation of the Ring. She was the oldest and Wisest.
Èowyn: Well, she's the character I feel closest to mine. She's human, brave and sacrificial (?). Her courage and bravery become inevitable for the downfall of Sauron when she kills Witchking. No man could kill him, so it had to be a woman.
Therefore all of them for different reasons.
