Dropbox is very fast and has probably the widest support but only gives 5GB free. Google Drive gives 15GB free with all Google accounts and has good support, but is not as fast. Google Drive used to be pretty slow but it's been slowly getting faster and is getting closer to Dropbox. I can tell what's fast and what's not because I have Google Fiber gigabit internet so I can really stretch out their servers.
I use 3 different cloud storage services extensively: 4 various size Google Drives including a 1TB account that's included with Google Fiber, a 8GB Dropbox, and a free 15GB Copy.com account. I haven't used Copy.com for long, but it seems good and at least as fast as Google Drive.
I'd say if you have enough speed on your internet connection to tell the difference and speed and/or support really matters, it's hard to beat Dropbox. Otherwise, if you have a Google account you already have 15GB free on Google Drive so you might as well use it.