There are a few apps that are PRIMARY apps for me. Whether it's on a iOS or Android. Now I'm on WP8 and I have them all as well, good quality as well. In actual fact, Evernote is astonishing on the WP8.
Here goes: SKYDRIVE and GDRIVE ( in the case of WP8 GDrive is substituted by INSYNC). MS OFFICE.
Evernote, IM+ (works better on Windows than on any other), Picasa Sync App (WP called Picasa Metro), News Reader (Nextgen on WP8, like Feedly), the social apps just to keep an eye out Facebook, Twitter and Google+), PayPal, XE Currency, and a good Unit Converter in this case the actual app is called Unit Converter and again, it's the damn best one I have ever used, so is ScoreMobile FC and Pocket Recorder. And there must be a great GPS and Maps.
Without wanting to brag too much,the Nokia GPS package (Here Maps, Here City Lens, Here Transit and Here Drive+) is unbeatable. And I was one that swore by TomTom and Google Maps. These Nokia apps are out of this world. Data Sense is important. That's an app that keeps track of your data consumption. It's not as good as the apps available on Droid or iOS (My Data Manager for instance) but it does what's necessary. CamCard Reader (it's the same app on all systems). Bing Finance is excellent, so is Bing Weather. Bing has News and Sports but I don't use them. Too US orientated. Scoff! Amazon Kindle is a must. I've recently acquired for all the OS's an app called FlightRadar and I tell you, it's become quite an experience using it. That plane that crashed recently at LAX (I think, or maybe it was SFO?) I followed it's flight path (the pilot had declared an emergency and so the app notifies you) until it belly landed at the airport. Amazing!