I'd say probably Jasmine. They have some things in common. When they're being insulted, they're both not afraid to insult back and will get ticked off. Eilonwy would admire Jasmine's strength in the face of adversity and try to be more like her since she didn't do much with the Horned King and the black cauldron. They could have nice chats about how hardheaded boys can be, since they both have had problems with their love interests.
Overall, I think Eilonwy would love Jasmine the most just out of a sense of admiration for her strong attitude and see her as a role model.
Uh, I know she isn't an official Disney Princess, but Alice. They are both strong willed, talkative, go off on random rants, both courageous. One thing they wouldn't have in common (and is almost opposite) is their trust. Eilonwy has some trust issues, while Alice believes everything. Oh, and they are about the same age, so they would really connect on a level that most 12 year olds wouldn't have with the other older Princesses.
