I purchased both Goodreader and PDF Expert back when I still owned an iPad and personally I liked PDF Expert better because (I think) it offers a bit more functionality. For instance, you can sign documents with it and it also allowed you to fill out PDFs that were designed to be filled out. (I mean it recognizes textfields and the like.)
But it probably comes down to personal preference anyway, so any of those will do and probably fulfil your needs.