OneDrive. Not because I love Microsoft. I do not. I just dislike Google more and it’s …the least inconvenient. I switched from Proton mail to Exchange before Proton offered file storage because I wanted a groupware solution. I’ve considered switching back to Proton but just haven’t had time to seriously look into it yet.
Edit: I just remembered the other hangup. Fucking Excel. It’s a monster but it can do things that no other spreadsheet program can do. Every once in a great while I need to use a bunch of VBA code or some obscure function that doesn’t exist in Libre Office. Most people aren’t going to have this problem though.