Would love to know the app who previously held the record.
Minecraft is paid.
I would guess FB Messenger or maybe Google Maps. Or something from China, seems like when QQ releases a core app there's millions who just go and download it.
Minecraft would be a strong candidate if it was a free app, but any price lowers your downloads drastically.
Games with a 2 in the name are the next most likely to have heavy first week downloads. Angry Birds 2 or Bejeweled 2 for example.