Using comparatives (more/most) with unique was frowned upon in the past because of the assumption that the adjective had only 1 meaning: ‘having no like or equal’. By that definition, something unique could not be compared with anything. However, up-to-date dictionaries list alternative meanings for unique, including ‘outstanding’, ‘remarkable’ and ‘unusual’. All of those adjectives are ‘gradable’: they can express degrees of outstandingness (eg most outstanding). In this sense, something can be ‘most unique’.
But in such cases, why not use most outstanding or most remarkable and keep unique for things that really are unique?