What should I do if a coauthor, who has contributed the most among my coauthors, declined to be listed as a coauthor?
I have previously asserted that a co-author has the right not to be recognized as a co-author, if that is her preference. If she is amenable to publication (as would appear to be the case here), then you may proceed with publication. (If not, then you would need to remove her contributions entirely, and then see if the paper is still salvageable in that form.)

However, you should make sure you have a written documentation of the coauthor's declining of credit. You should also make sure that you have explained clearly why you feel she should be co-author; perhaps you can include a list of the contributions of the other co-authors.