Pages,numbers and keynote are of course installed first,as it is apples own document apps.Pages
Numbers
Keynote
Spotify
Amazon Vídeo (if you have prime)
Something to unzip files (I've been using GoodReader)
Apart from those anything that helps you do what you do with your iPad.
Apart from the ones I mentioned above I also use...
Calculator
Weather
XE (for currency conversion)
Translator (google)
A couple of alternatives to Safari (Firefox, chrome, etc)
Google Maps
Kayak (flights, booking and tracking)
It all depends on what you use the iPad for.
Thanks! I've got the normal 2017 iPad now and actually didn't have any of these on here. I'm a Physical Therapy student so using it for school related and note taking/highlighting is my main use, currently other than just for recreational use.