I'm reading Frankenstein - the 1818 edition - but this one is special. Mary Shelley marked up a copy of the 1818 edition with changes she wanted to make for future editions and she left that copy with her friend Lady Thomas in 1823. That book is now in The Morgan Library in New York. In 1974, James Rieger published an edition of the 1818 text, with Mary Shelley's 1823 changes (indicated in brackets).
This is a groundbreaking edition for a number of reasons. It is (or was at the time) recognized as the definitive scholarly text, and, at the time of its publication, all the other editions of any significance reproduced the 1831 edition. It's interesting what has happened since: scholars, especially feminist scholars, have signaled a preference for the 1818 edition. So now the 1818 edition is widely available and is often used in courses.
I first read Frankenstein at uni, and we read the 1831 edition, which I loved, and that's the edition I've reread multiple times. I'm kind of biased in favor of that edition. To me, it is the novel. I only learnt that there were two main editions that are quite different fairly recently - a few years ago. Or I might have known but I didn't think it was that big a deal. I think I've only read the parts of the 1818 edition that are different from the 1831 edition, just to get a sense of the difference. I haven't read it through as a whole, so it's exciting to be doing that now. Maybe my view will change, and I will come to a new appreciation for the 1818 edition.