There are really huge debates to be had here in regard to the definitive text but, according to my assessment it's the 1922 edition of Joyce's work that is most clearly now a part f the public domain and so that was the place to go for my sources.
I'm treading lightly on subjects like this, for obvious reasons, but it seems clear to me that one of the points of current public domain and fair usage laws has something to do with allowing artists to respond to earlier works of art that have, through history and reader involvement, wormed their way into the public consciousness. Joyce's work is iconic. Hans Gabler's is wonderfully, richly-researched and academically exhaustive, a great boon to the readers wishing to understand a text that has had many disturbing permutations, some of them cased by the author himself. But Gabler did not reinvent the novel and it has lived as an iconic piece of fiction for many, many decades before he took up the task of "correcting" it. I'm happy for the things his research has taught me about the novel, but, looking at his contribution as a uniquely new work of art, and therefore re-packaging the existing public domain laws so that the copyright starts over again in 1984 with this "corrected" text completely ignores the life this book has already enjoyed as an individual artistic statement.
There are numerous problems with the 1922 text. That's clear. But this is how the work came into the world, "warts and all," and this seems the best text to represent it's effects on us all as readers.