Depends what you want to use it for really. But I am sure you know that all are not fully featured desktop apps. I mostly use for review, comment and presentation purposes, rarely creation or serious editing.
I use the native apps (the filing system is wonderful and iCloud syncing a total joy), combined with Goodreader (for unzipping files and general pdf and document viewing) and Notability.
Keynote is fabulous, the one I use most often for viewing and simple editing of ppts.
Numbers is good for simple spreadsheets, anything with lots of stuff embedded is going to not work though.
Pages I find a bit rubbish, it gets tripped up in fairly fundamental ways by almost every doc I put in, but then we rarely use word docs.
You can annotate pdfs in goodreader, but it works better in notability. I find that a real killer feature of iPads, which is a big part of my job, and something that really can’t be done in any sensible way using any other system. Wonderful.
HTH! 😀
Kev