It's difficult to rank any one character as the one she "likes/loves the most," since there's more than one way to categorize emotions felt. It's not very clear-cut.
1. I'd have to say her parents come first, since family love is often the strongest. Blood relatives are unique in that they're related to you in a way no one else can ever compare with. You'll only ever have one set of parents, and that's that.
2. Fry snags a close second, however, since he's her romantic interest as well as a true friend. Those are rare to find and often deeply felt for.
3. Nibbler places third, becauseI'm sure all pet owners know how attached you get to a beloved animal. And he's so darn cute.