This isn't in order of what I like, this is the order they are shown in the movie.
1. The Rags
Am I the only one who likes this? I think it's cute how she is showing off her dress like she thinks it looks good. Of course it looks ridiculous when it's all bunched up, but after it smooths out, I think it's pretty. White is probably the best color on Ariel (besides her green fishtail and purple seashells). I wouldn't like it as clothing, but it's the first thing she wears, and only for a few minutes, I think it looks good on her.
2. The Pink Dress
Of course this one clashes with her hair. But I think it's good that they did this in the movie. The fact that they had a spare dress in the castle, I think it subtly shows that they didn't expect a girl with bright red hair. I actually like the way it looks, and I think it's ok that she wore it for a little bit.
3. The Pink Nightgown
This one is cool because Ariel looks really cute and innocent in it. It's simple, loose-fitting, and flowy. I don't have much too say about it, but I think it matches the mood each time we see her in it.
4. The Blue Dress
I really love this one. I love the way it twirls when she dances. I love the length of it; it cuts off above her ankles, which I think is pretty during the scene where the shell necklace breaks at her feet. I think the color is fine. There's only one part of the movie where her hair is bright red while she's wearing that dress. That's during the tour of the kingdom. When they are in the lagoon, her hair looks darker and it matches the dress better. When they crash the wedding, the light of the sunset changes her hair color again, and I think it matches decently. I probably like this dress because it looks like Aurora's peasant dress, which is one of my favorites (I'll write another article about my favorite DP dresses later). I also like the bow she wears with this dress. I think it's adorable on her. And I like how she's not wearing the bow during the more serious scenes.
5. The Sparkley Dress
I love this dress. You only see her in it for like ten seconds, but I think that makes it even better. It's pretty, sparkley, twirly, and I like the style of it. It shows her shoulders, and I think that style suits her very well (of course because she's a mermaid).
6. The Wedding Dress
I don't mind this dress. During this scene, I'm not focusing on what she's wearing. The white color looks good on her like I said before. I do think the puffy sleeve things are kinda big, but again, I don't mind. I think it suits her, as a lot of things about Ariel are big: her hair, eyes, the blue bow, and her personality. Ariel's really happy in this scene, and that's what I see, not her dress.