IFTTT, Saga, VSCO, for now. And maybe Withings.
VSCO especially because filter recipes would be developers trade secret.
Would like to ask for Hatena too, but since Windows Phone's present in Japan is as good as Blackberry (no new device available from carriers), I'm pessimistic on this one. The service hardly has any presence outside that island country.
The most important 1st party apps is for services that doesn't have API for 3rd party to do it. Quite satisfied with the 3rd party Foursquare, Flickr, Instagram, Newsblur, Pocket apps.
Flipboard and CamiApp is already available for Windows RT. Microsoft should nudge them to update it to universal apps to cover both platform.
Tumblr, only want them to update feature to give notification and show stats, as well as performance increase.
And update the social service applications to link to the People hub.