Bare in mind i am based in NZ and our internet infrastructure is shocking so cloud transfer can be sloooooowww as! but these are the tools i have used, some come with pros and cons
for that one off file that doesn't need to be archived and is reasonbly fast definately recommend We Transfer
WeTransfer
Not my favourite but this cloud space has api built in with so many different places and built into phones and computers now, for ease of use amazing, but its pricey especially when working with multiple users
Dropbox.com
for security this is great but sometimes the security is so good the end user gets stuck, off course built into microsoft products and office cloud, but end user always seem to have a hard time when sharing from Microsoft to Microsoft this is great
Sign in
this is my go to most of the time end users has a great experience and the interface is what you would expect and simple to navigate built into most phones and easily accesible from chrome, however there can be some security issue if you don't check ...
Google Drive
Amazon is not everyones favourite but i rate it for ease of use, cost and end user experience definately worth a try
AWS
Security is great, even if you loose your acccess details it can be difficult to get back in but you get lots of free space end user is happy and has the benefit of computer and phone access
