I would have to say Road Trip and Hair Salon 2. Only because both keep my 8 year old girls entertained for hours. That's priceless on those long road trips. But runner-up would be Plex. That's my entertainment.
I use my phone for business and entertainment. My best purchase ever is FileBrowser for Business. Makes your iOS device so much more capable. If I could only have one app this would be it ( the standard paid version is very close in functionality). Brilliant support and continuous updates.