According to Markus:

BestRank should not be a possible value for wikibase:rank. Instead, it should be made into a class and used like wds:id rdf:type wikibase:BestRankStatement. One reason is that “best rank” is not a real rank, so it should be treated differently. Another reason is that we should keep the distinction between given data and derived data. The best rank information is derived from the given rank information, but it can change even if the statement is not changed at all (because other statements have changed).