The definitive edition seems to be that published in 1993 by the Naval Institute Press as translated by Walter James Miller and Frederick Paul Walter (ISBN 978-0870216787 paperback) which is described as "The Completely Restored and Annotated Edition".
Frederick Paul Walter's original 1991 translation is available as "Twenty Thousand Leagues under the Seas" as a hardback edition (ISBN 978-1904808282 List Price: $39.95) published by Evertype in 2009. This hardcover edition is viewable on "LOOK INSIDE!" with Amazon. There is also a paperback edition (ISBN 978-1606641880) which was published by Aegypan in 2008. But beware: the "LOOK INSIDE!" for the paperback edition is actually of another version published in 2007 by BiblioBazaar (which looks like it was typeset on the cheap).
Frederick Paul Walter's translation is also available as "Amazing Journeys: Five Visionary Classics: `Journey to the Center of the Earth', `From the Earth to the Moon', `Circling the Moon', `20,000 Leagues Under the Seas', and `Around the World in 80 Days'" (ISBN 978-1438432380 List Price: $34.95) published by Excelsior Editions in 2010. This paperback omnibus edition is viewable on "LOOK INSIDE!" with Amazon. Note that the stories are printed like a newspaper with two columns to the page.
William Butcher issued a new translation in 1998 and revised it in 2009 (ISBN 978-0199539277 paperback) published by Oxford University Press with the title "Twenty Thousand Leagues under the Seas". This is viewable on "LOOK INSIDE!" with Amazon.
All of the other myriad editions seem to be re-issues or adaptations of the first English translation done in 1872 by Lewis Page Mercer (who cut about 20% of Verne's original text and made hundreds of translation errors).
The choice between the translations by Frederick Paul Walter and William Butcher might well come down to a matter of style - rather than accuracy in translation - or a preference for a hardcover over a paperback edition. I own the Naval Institute Press edition. I have seen the other editions on "LOOK INSIDE!".
Frederick Paul Walter's Evertype hardback edition (ISBN 978-1904808282) is typeset in "Fournier MT" which gives the text something of a nineteenth century appearance. There are also black-and-white illustrations which are of the period (but not necessarily thereby of great artistic merit). The dustcover of the Evertype edition shows - for some inextricable reason - a modern photograph of a stingray (where a squid, at least, might have had more relevance to the book).
If I were just interested in reading the story I would probably buy the OUP edition which - like my copy of OUP's "Around the World in Eighty Days" (also translated by William Butcher) - could be expected to brown-up within a year or two as it's printed on cheap paper (the same as the Penguins of old). If I were buying it as a gift I would probably be tempted to select the Evertype hardcover translation by Frederick Paul Walters (even though I don't know what quality of paper it's printed on or whether the pages are sewn together in quires or just glued together like a cheap paperback).
With the benefit of a little high-school French you can easily compare your chosen English translation with an online original French text.
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