Cinderella in my opinion is the strongest princess, closely followed by Rapunzel, but demonstrating her emotional strength and endurance on a more profound level: she was exposed to the disturbing and traumatizing truth of her step family not loving, caring or taking her feelings and her entire existence in consideration beyond methoically and deliberately expoiting and using her as a servant, never feeling even the slightest of concern or emotional attachement on their part. Rapunzel at the very least was provided with an illusion of comfort and love and had it in her disposal for 18 years, although it must have been a thousand times more damaging and shocking to find out that your caring and slightly overprotective mom was actually a person who intentionally negatively affected your psychological condition and systematically emotionally pressured/abused you in order to destroy your self worth and worldview for 18 years for the sake of keeping you isolated and deprived of any basic human contact or experiences all for entirely selfish purposes than getting away from initially established and blatantly evident abusive relationship where you can rationally examine and analyze the situation recognizing the extent of oppression. Still I'll go with Cinderella because while Rapunzel's strength was more prominent in regards to her journey of self discovery and escaping oppression facing numerous obstacles primarily linked to the astonishing amount of insecurities, fears and misguided concepts of reality she had to consequently and eventually work through, Cinderella had no choice other than to be strong and keep on holding on to positivity and hope practically since childhood and up to her late teens. And Charming did not randomly and conveniently stumble upon her house providing an excellent opportunity for her to have an overwhelming adventure, instead she actually had to go the the palace on her own and get him.

Mulan,Cinderella and Jasmine too

Or Pocahontas

Mulan :D She had the courage and strength to protect her family's honor and her father. To me she is the strongest and the girl I look up to the most! She fought as a boy for goodness sake! THAT TAKES GUTS AND DETERMINATION XD

I HIGLY agree with @Maryksand

But when I think about it, I begin to consider Cinderella too.

And Cinderella

I don't know if you meant physically or mentally but physically I'd go with Mulan.

Or Pocahontas.

Physically maybe more Pocahontas.

I agree 100% with maryksand, thank you for such an amazing comment.

also Mulan and Pocahontas.

She has so much inner strength and resilience....