Is this even a question? Haha. :)

This was close for me with Gryffindor but ultimately, I see Gryffindors as being very heroic and more likely always doing the right thing from the start as well as being super virtuous (not wanting to hurt a fly type). Also Merida meets 2/3 traits of Gryffindor provided but all 3 for Slytherin.

i think she's true mix of Gryffindor and Ravenclaw - bravery, daring, learning and curiosity

Layla: But Cormac McLaggen wasn't heroic and acted slimy and really obnoxious, not heroic and virtuous. Also, Romilda Vane tried to drug Harry with a love potion. That's not heroic or virtuous at all! Merida is reckless, outspoken, brave enough to fight off her dad and the others when they were trying to kill Bear!Elinor, and selfish, but eventually willing to own up to it. Those all seem like Gryffindor traits to me. :-)

I think she is Gryffindor. I also think that Gryffindor shouldn't be idealized. Not every reformed jerk should be in Slytherin. I really dislike the fact that the nicest Slytherins in canon are reformed jerks but not all reformed jerks in canon are Slytherin. James Potter and Sirius Black were quite flawed people and weren't always just. Even Ron was mean at times.
I really dislike this "Slytherin = making serious mistakes and being a reformed jerk at best, Gryffindor = being good all along" stereotype. I believe that Tiana is much more Slytherin than Merida, while she is also wiser and more virtuous.

Probably a Gryffindor.