I use Amazon Drive. It comes with amazon prime. It's great especially for storing photos - as it supports dumping original raw files as well. The downside is that they stopped supporting Dev APIs.
My usecase is something alone the lines of building a product/hosting solution that allows users to place files on their drive and I'd modify/sync/hotlink it to a cloud hosting server. Amazon Drive seems a good candidate too, I'm simply looking for the most popular one that users use and one that has decent API support for download/uploads.
What is your particular use case?