Dropbox's big limitation is whenever somebody shares a folder with you, that folder counts towards your personal storage limit. It's pretty weird logic. It really nails you when you're on their free account and then you get shared a couple massive folders for business.
I use both Google Drive and OneDrive (I have a 365 subscription). I back up my critical documents across both, but use OneDrive for the bulk of my files (I try not save anything locally on my laptop). No real difference I've noticed between the two.
Some additional insight: OneDrive's mobile application will automatically optimise whiteboard photographs that you take on your cellphone if you've got it sent up to back up your photos automatically (handy for business), while Google Photos will automatically apply all sorts of other cool edits (e.g. filters, panoramas, gifs and even snow effects). The big difference is that Google Photos automatically compresses your photos before uploading them, while OneDrive uploads them at full resolution.