As said above get the google stuff, Drive/Photos for cloud storage/backup, docs/sheets etc if you have the need.
Couple of my must haves not mentioned above…
Yr.no for weather
Navmii for offline navigation
ES File Explorer for file management
Then it’s really down to what you want to use it for, there are apps for almost anything.
Once you get a bit more settled into it you might want to look at some of the automation apps. Tasker is the big one but while very powerful it’s not so easy to get going, AutomateIt and MacroDroid seem more user friendly for the basic stuff.
Edit: Widgets….
For home screen calender widgets I like Business Calendar, but don’t like the actual app, prefer Google Calender.
Power Toggles for settings/control buttons
I’m just trying out Transparent Weather Clock widgets on my new phone, think it’ll be staying.