For taking handwritten notes, drawing, annotating slides, etc. I use Notability. I reviewed a few of these apps and liked it because it has a macOS version, offers the option to store all files on a local WebDAV (no cloud needed), differentiates between finger (movement) and pen (writing) and is reasonably simple, i.e. quick to use.
I also looked at Evernote and OneNote. Both are fine programs and I am ok with having most of my notes on their servers; just not all of them. OneNote lost a bit lately. It used to have local handwriting recognition. This is now in the cloud. Impractical and a privacy issue.
I also looked at the MS Surface 4 instead of the iPad but found that I rather have a oversized tablet then a laptop with a detachable keyboard. I enjoy a selection of excellent epub readers and touch enabled video players more than a RAW workflow (Darktable, etc.). These were the things I found being much better on the iPad or on the Surface. There are for sure more differences, depending on the user's needs.
On the iPad I also find PDF Expert for PDK journals and books and Hyphen or Marvin for epubs quite useful. Using Working Copy as a git client is nice for occasionally looking at code or keeping notes in ASCII text. Surprisingly typing on the large 12.9 inch screen works well for me. This was not the case with smaller screens.
The iPAD is the paperless office for me as well as a portable library. It could do much more I think. Working on it.
