I use all of them (Google, OneDrive, Dropbox) due to various needs, but Dropbox is my primary account too.

I've found that inSync is a wonderful client for GoogleDrive and OneDrive for all major platforms. It has some nice features like out of tree syncing (sync a cloud folder to anywhere on the filesystem and keep it synced), and .gitignore like ignore syntax.

I'd recommend it heavily. It's a one-time-payment application with no limitations.

They've added dropbox support recently.