I mostly use Resilio Sync besides Dropbox. It works really great: fast syncing, nice user interface, etc. What keeps me from fully switching is that I hit some nasty bugs in the past (e.g. all folders suddenly getting disconnected, etc.).

I recently signed up for Sync.com, due to their prioritisation of security features and they seem like a good company.

I also tried sync.com recently. Unfortunately, the client did not look very native on macOS (poking around revealed a lot of .exe files, so I guess they are using Mono). Besides that sync was quite slow compared to Dropbox.