For sheer convenience I use Drive which comes with 15GB of free storage to back up everything (I actually have 65GB through whatever subscription for two years but when it runs out I'll worry about that then!)
My personal preference from a useabilty and UI point of view is Dropbox but that only comes with 2GB of free storage so I only store my most accessed files on there. With regards to app data your Google account is supposed to take care of that (providing you've got it set to sync), but I've found it to be rather hit-and-miss at times. Of course, your ultimate backup - app data aside - is the hard drive on your PC, which of course you have a seperate backup of as well (don't you?)