Maybe Dropbox's is the best out of a bunch of mediocre syncing solutions. However, I do wish to add that Dropbox struggles with scale: lots of small files slow down sync to a crawl and spin CPU at 100%.

Concretely, I had a certain folder with 100,000+ small files for some research I was doing with a collaborator. It took my laptop 24 hours to sync this folder, and throughout my CPU was at 100% throughout. For comparison, rsync got the same job done in a few minutes. See https://help.dropbox.com/accounts-billing/space-storage/file... , the above terrible experience is in line with this caveat.