Best for mostPenguin (Mitchell) · 2003
The Penguin Classics edition is the right Great Expectations for most — readable, well introduced, and it prints Dickens's original (darker) ending in an appendix alongside the revised one he published. Good notes, low price. This is where I'd start.
Best for studentsNorton Critical (Rosenberg) · 1999
The Norton Critical Edition, edited by Edgar Rosenberg, has been called 'an editorial masterpiece' — the definitive Clarendon text, the discarded ending, Dickens's working notes, and full criticism. The one to own if you're studying the novel.
Scholarly alternativeOxford World's Classics · 2008
Oxford World's Classics gives you a scholarly text based on the Clarendon edition, with Margaret Cardwell's introduction and notes. A strong alternative to Norton, lighter on the criticism.
Charles DickensGreat Expectationstr. Free (public domain)
FreeFree (public domain) · 1861
Great Expectations is public domain and free. Free and complete — though note that free files usually give only the revised ending, where the print editions include the original too.