I'm really liking Notability. Key features for me include Dropbox back up (in PDF if you choose), the ability to import and annotate PDFs - as well as add pages, the various paper types, and the ability to cut/copy/move parts of your handwritten notes if you wish (really handy when annotating diagrams which then get expanded!). Plus there's a Mac app where I bulk add PDFs I'm going to use later (past papers for exams usually).