Cloud storage is a necessary evil these days if you're using multiple devices. For iOS, it's crucial as there is no standard file system for the OS. If you're saving, sharing or even editing files on iOS, your choice becomes even more important. In the past year or so I've used all of these. I'll breakdown which ones are best when using multiple iOS and macOS devices.
The Best: Dropbox
Dropbox is the best overall pick for syncing, sharing and editing files. Syncing is quick and seamless. Dropbox is the most widely supported service for third party apps to sync files. Saving a file is extremely easy on iOS. There is a "Save To Dropbox" option directly in the share sheet. From there you can easily navigate to any folder to save your file. Sharing is also super simple on both iOS and MacOS. On MacOS you can simply right click on a file in Dropbox to share a file. It's just as simple on iOS to share a file or send a link to it. It's funny, the main reason I'm giving Dropbox the number one spot is the simplicity of sharing a file. All the other services don't come close the simplicity Dropbox offers. It also has some great features for previewing files directly in the app. Much better than what the competition is offerings. Dropbox offers a pro upgrade for $9.99 a month or $99 a year which will give you 1 TB of space. You can rack up a good of amount of free space on Dropbox by doing a few different things .
So Close But Missing Key Features: iCloud Drive
So although I'm listing iCloud Drive as the second best option, this is actually my preferred cloud storage service. It's integrated into iOS and macOS so no extra software to install. The Photos app is able to utilize the space on iCloud Drive to store all your photos and in my experience it has work flawlessly although sometimes a bit slow. I hear many complaints of syncing issues but I haven't experienced any issues with file syncing between devices. It's not as quick as Dropbox but still gets the job done. It's third party integrations are very good and getting better as more apps offer an iCloud Drive option. Some of my favorite integrations include syncing settings in Airmail, never losing my place in Tweetbot and saving / editing files from Pixelmator . Overall I would say iCloud Drive offers the best overall experience on iOS. There's one HUGE problem...you can't send someone a download link to a file or folder from the app on iOS or finder in macOS. This is crucial for me and the only reason I can't recommend it as the best choice. You'll need to send the file to another app like Droplr to do that. iCloud Drive has a sweet spot for pricing too. For $2.99 a month you can get 200GB of space. Most users will find that is enough. There's a 1 TB for $9.99 and 2 TB for $19.99 a month as well.
Honorable Mentions: Google Drive, OneDrive and Box
Here's what I'll say about these services, they will mostly get the job done. They don't integrate as well as Dropbox or iCloud Drive and for the most part are not worth the cost on their own. If you live in Google apps, Google Drive is probably your best option. Their iOS and macOS app are sufficient. Google Drive in my opinion has the best and fastest web interface which is expected. Search is also great but not as many apps integrate with Google Drive . OneDrive has improved a ton recently but the only reason you’ll want to use this is if you’re already paying for an Office 365 subscription because it's included. It has some nice integration with Word , Excel and the other Microsoft Office apps but nothing makes it stand out on its own. Box seems to care more about small business and the enterprise although the service works very well. Box has a file upload limit even on paid accounts which the main reason I can’t recommend it.