Or gryffindor.

she defies everything to get what she wants...so i guess

In the end, everything Ariel does is for herself. There's not really one moment in the entire movie that where does something to help another person out, and Flounder doesn't count since she put him in that situation in the first place. I guess saving Eric could count but it's something I'd expect any normal person to do, especially considering she lives in the ocean and has a clear advantage. But regardless, while she might be brave and interested in the human world, every decision she makes is motivated by her want to become human, and she's willing to do anything she has to to obtain that ambition.

To put it this way, if Ariel had discovered earlier on that her father's Trident could turn her human I think she'd definitely steal it and attempt to change herself. Her selfish wishes overcome any other trait she has.

Definitely Gryffindor because although she can be impulsive and reckless as well as incredibly intellectually curious, she is mostly brave, chivalrous and daring. She pushes up and out. A lot of her misbehavior does admittedly stem from the fact that she is beyond all of her peers. She was ready for a bigger and more complex world than all of the other mermaids. Above and beyond makes her a Gryffindor in my book.

Ariel is not anymore selfish about her dreams than Merida or Tiana. So the above argument just doesn't make much sense.

AS for saving people, Tiana let Ray DIE on her watch, and Merida turned her own mother into a bear, which she knew her father would attempt to kill. So, as for thinking only of themselves, Tiana and Merida FIT that category, too.