I like Celestia and Luna equally. I find Luna's unintentional intimidatingness and social awkwardness cute, and I have a soft spot for her underdog status compared to Celestia. As for Celestia I like how she's all princess-like, with her being caring, wise, and generous, yet isn't afraid to break out of that mould (like that prank she did at the party with the tea cup).

Looks-wise, I find them both equal as well since Celestia's light and colourful colour scheme and Luna's dark blue and black colour scheme are both quite pretty.

To Proto: the prettiest and cutest question wasn't just for the mane six, and it didn't necessary have to be narrowed down to one or two. I had five for prettiest in my answer.