Ok, so you said between Alice, Esme or Bella, but I'm going to add Rosalie too in my list, sorry.
First one: Alice, definitely. Ashley Greene is gorgeous. Need more?
Second one: Rosalie. Though I think they could have found a prettier actress for her(because in the books Rose is described as the most beautiful creature on earth), I still think Nikki is quite pretty, and she was definitely prettier in NM. And she looks great with brown hair too.
Third one:Esme. She's pretty, I guess. I like her with curly hair in Twilight, but it was really a good choice to make it straight in NM. She looked prettier, and more like a vampire(in Twilight, she didn't seem like a vampire, at all).
Fourth one:Bella. In the movies she looks horrible. I'm sorry, but it's true. The moment I saw her in Twilight, I thought "Oh, god, how the hell does she look?!" And it just got worse in NM. She looked like hell. And her clothes! I know Bella isn't fashionable, and I'm glad they stayed true to the book, but enough is enough. Real life is an other story. I wouldn't say Kristen is very beautiful, but she sure is pretty. And she looks really great on photoshoots. But for me, she's still the least prettiest girl in Twilight.
Others, like Victoria, Jessica or Angela are pretty too, but I didn't want to include them on my list.
So yeah, Alice wins for me!