I would say either Jasmine, Ariel, or Rapunzel. Jasmine was the first POC, Ariel introduced a really new type of personality, that differed a lot from the classic DP's, and Rapunzel brought a new form of animation.

It's part of the reason why she's my favorite, because she's so different to most of the others, in personality, and she doesn't have the frisky, whimsical, childlike, playful and/or adventurous demeanor that most of the others have, but at the same time she is a dreamer, while being more grounded than the others. Also, Jasmine stands out a lot, but they all have at least one quality that makes them stand out, except maybe Belle and Rapunzel.

I'd say her or Ariel.

None of them "stood around for some guy" and Merida is definitely not the first to not want a prince. this was pointed out dozens of times! Half the girls don't marry at all, and Pocahontas was the first to just plain not end up with someone. I'll never understand why people seem enamored with the idea of women not "needing" a man, but get cry sexism if it were the other way around.

Anyway, she's the one who started it all. she paved the way for every single heroine following her, and has all the traits people think of when they hear "Disney Princess". Her optimism is unmatched only by Dory, and she stands out pretty starkly next to heroines today, despite her having many modern ideals.

I'd also say Ariel (Disney going in a new direction), Jasmine (first nonwhite princess and the first to have real negative flaws), Tiana (the first anti-Disney Princess), and Merida (the INTENTIONALLY very anti-Disney Princess/girly girl/ends up in a romance/etc), but they all add something unique, except maybe Rapunzel, who has no new traits at all. (Mulan was also the first insecure girl.)

She was the first. At the very core, all the other Disney girls are simply twists and adaptations of the Snow White formula. Variants.

Jasmine or Ariel. But Ariel really set the stage for the princesses to follow.