My mix-and-match approach
I have a 50GB iCloud account, and use iCloud for my core services. My calendar and notes are on iCloud, and I greatly value having (mostly) seamless access to these between devices. I use Messages and FaceTime as my primary chat and audio/video call apps. I also use iCloud as the primary backup for my iPhone and iPad, though I also do occasional encrypted backups to iTunes, especially before upgrading devices.
I use Google Photos to automatically backup my photos. I have the Google Photos app running on all my devices – iPhone, iPad and Mac – and find it fantastic to have unlimited storage free of charge. To me, this is an absolute n0-brainer.
But my primary cloud storage service is a 1TB Dropbox account. On my Mac, my Dropbox sits at the top-level in my account folder, and everything else is stored in sub-folders within this. This means that all of my documents and media are available to me on any device, including on the web. I’ve used it for several years now, and as I said earlier, have found it 100% reliable and super-fast to sync.
Finally, I’m a huge believer in belt-and-braces. When I’m working on something important, like a novel, I copy it to multiple places. It already sits on Dropbox automatically, but I also make manual copies to iCloud and to my (free) Google Drive account.