Personally, its not any one app that is missing. It's the blatant disregard for Windows Phone by 1st party developers. I was talking to a co-worker today about this and he made an excellent point. It has become "cool" to shun Windows Phone as a platform. It's almost like an ongoing joke on the Windows Phone community to neglect them. The most frustrating part of this is that market share is so small that they can continue to operate that way.
I'm a huge fan of Microsoft but I'm concerned for the future of Windows on phones. This strayed from the topic a bit but I think its still relevant. Anyways, to answer the question, I'm missing Snapchat, Google services, Amazon services, Houzz and more important than the apps themselves, continuing support through updates and embracing the Windows look and feel.