OneNote mostly. The iOS version isn’t as full featured as the desktop one (it’s more in line with the Windows Store version), but it’s worked well for me. That’s where all my notes were when I switched so it seemed easiest to stick with it.

The built-in Notes app from Apple has also gotten more capable in iOS 11, and there are other options that I hear good things about but haven’t tried.

https://www.imore.com/best-notetaking-handwriting-apps-apple...

Procreate and Autodesk Sketchbook are both great for more powerful digital painting / drawing, I’m personally in the Procreate camp.

I've been using Good Notes and the handwriting part of it is fantastic. The notebook management part of the app is somewhat weaker though and lately I've been thinking about trying something else.

For sketching quick diagrams on meetings I love Linea.