Recommended Cloud as no AWS S3 available

I have 15000 PDFs in a hierachical stucture on AWS S3. As there is no cloud attachment for S3, I can use Dropbox, Google Drive or One Drive. What is the fastest, best sync functionality, easiest to use and then but not the priority, most cost effective. (80GB Data currently). I have purchased WP File Download and the add-ons and your documentation and support looks very good. please can you assist me in making this decision on the cloud storage. I have read the documentation and I would prefer not to have to test each option. Do you plan to include AWS S3 in the future?

thanks
Simon