Envy (Latin, invidia), like greed and lust, is characterized by an insatiable desire. It can be described as a sad or resentful covetousness towards the traits or possessions of someone else. It arises from vainglory,[35] and severs a man from his neighbor.[5]

only because she covets what her stepsisters have and the life at the palace.

I'd say her. She's very envy about human world and imagine how humans have better life than hers.

She thinks whatever that she has not recieved is better

Or Ariel.

She envies the humans I guess?

Towards humans. Also Tiana and Cindy.

I dont see Ariel to be envious at all. She was curious about wanting to live another life than hers. She could see human world mostly from underwater and thats why craved for it, as it was so different from hers.
I find Jas and Mulan and Belle to be most envious bcoz they already had a decent life, a life they could really like if they put effort into but they wanted to live like someone else despite of the comfort of their existing life, plus the life they wanted was no drastically different from theirs but they just thought of it being better.
Jas the most, bcoz she literally was living most luxuriously and craved for something out of pure boredom.