Which characters do you think are the most unreliable narrators, either in thought or in what their words imply?
I mean, anon, that's kind of the point.
All of them are unreliable in that they have biases, have been through certain events, and have limited information. None of them is any more objective than the other (for all they claim to be) which is why we need all of the ones we see to tell the story.
Lyra is at first enchanted by Mrs. Coulter as well as Lord Asrael then horrified of them and never has a clear understanding of their feelings, their objectives, or anything about them.
Will is a hardened, overly suspicious, boy who is still a boy at heart and desperately wishes to believe that there was a reason his mother was the way she was and that it was something beyond her control. Not to mention the mixed feelings over his father.
I wouldn't say any of them are more unreliable than the other because they all, again, are in the same boat of: have their own perspective, their own goals, their own biases, and their own limited set of information they're working with at any given time.