To me, the biggest question in selection of a cloud service is “what are your needs as far as security / privacy?”
If you plan to store only data that requires minimal security, then Dropbox is certainly the 500 lb gorilla and has, as far as I am aware, the most and best integration with various third party apps and services and has excellent integration into MacOS / iOS.
My big problem with Dropbox is that there is no real security, because files are stored at the Dropbox end in a manner that allows Dropbox to decrypt/access them. While I am not storing the secret of cold nuclear fusion (obviously I keep that on local storage), I do have things in my shared folders that I don’t want to risk becoming accessible outside of my control.
My solution has been SynologyDrive, which I have found to be highly reliable and I have been using if for some time. ResilioSync is also very reliable, and I own a lifetime license and used it for quite some time as well. If I did not have a Synology I would use ResilioSync. If you want to roll your own cloud, you can easily use ResilioSync with a Raspberyy Pi or just with your desktop and portable Macs.
There is a downside to the use of roll your own cloud approaches. The very fact that you “own” your service also means that there is no real cloud server - for example, if your house burns down and your Synology, desktop, and laptop are al in the house, you lose everything, while if you had your data on Dropbox you would have the cloud version for backup. I address this by having BackBlaze running on my iMac Pro, and all folders that are synced with SynolgyDrive are backed up to BackBlaze so I have an offsite backup of everyihing.
I tried Sync.com, which is a Dropbox type of service with end-to-end encryption and all data on their servers is encrypted with a key you hold, so they cannot access your data. The problem with Sync is that it does not handle MacOS metadata such as tags, and that was a dealbreaker for me, so even though I got a really good deal on a 2 TB storage deal, I had to cancel my account with them.
In addition to SynologyDrive, I also do run a WebDAV server on my Synology which I can use for syncing DEVONThink. Right now, OmniFocus syncs with the Omni servers. I suppose I could look into moving that to the Synology as well.