The other iPad heavyweight, and the pick for lectures and meetings because of one feature: it records audio while you write and ties the recording to your notes, so tapping a line replays exactly what was being said as you wrote it. For students that is close to magic.
It overlaps heavily with GoodNotes on handwriting, and the choice between them is largely taste, plus that audio-sync feature. If you take notes while someone is talking, Notability is the one.
