I don't like DropBox, it has way too many "confliceted copies", but the final straw came when it somehow managed to overwrite newer code with older, costing me weeks of effort. Plus, it is not encrypted on my device before sending to their server.
I used to use SpiderOak, which is encrypted on my device before sending to their server, so that they cannot access it.
Alas, I lost my password and it is so secure that their support can't help me, so there go my 15gB, which I got from referring friends.
I am looking for something gratis to synch some files of my choosing between my Windows laptop & Android 'phone. Mutatis mutandis, I would prefer the one with most free storage (extra storage through referrals would be a bonus).