Comparative form is lieber. This is how we study usually :
gern, lieber, am liebsten
On this site , it's given: https://www.verbformen.com/declension/adjectives/lieb.htm
lieb, lieber, am liebsten
So, which one is the correct version?
Additional question, lieber means prefer and rather. Hence, I think it is adverb. But, I have come across the question and answer in B1 Netzwerk book wherein lieber is considered as adjective in comparative form.