What you really need to be asking is which citation service gets indexed by Google the best. the citation game has decreased in power due to Google's newer unique content algorithms. API aggregators will be the worst. that used to be considered the best. I'm still looking for an API aggregator that can guarantee indexing but I think the only way is to go manual. that way you can at least write unique descriptions for a lot of the directories and increase your chances of getting them indexed. the logic is simple if you have a hundred directories that say the exact same thing Google Sees no need to index any more than 15 to 20 of them. a lot of people make the mistake of thinking oh look I'm synced up with 100 directories and there's my page. meanwhile 85% of those aren't even indexed and therefore irrelevant.