Lynch officially only created the theatrical release (137 mins). There are several variants of the "TV version"/Extended Edition, which runs about 180 mins (depends how you count some non-movie materials). Most of this was restoring edited-out footage (the movie studio wanted something about 2 hours) and showing colored storyboards with narration instead of the Princess Irulan intro. Lynch has disavowed all other versions beyond the theatrical. IMDB notes
Theatrical version is 137 minutes long; TV version seen both in syndication and on most cable networks, prepared under protest from (and eventually disowned by) director David Lynch, is approx. 176 minutes long (minus commercials) and features outtakes, additional footage, test close-up shots of certain actors, and even fabricated (i.e. "cheated") footage (made up of repeated stock footage from certain points in the film to make it appear that footage had been added--one reason why Lynch took his name off the credits of the TV version). The TV print credits 'Alan Smithee' as director. Whereas the theatrical release features a brief introductory narration spoken by Princess Irulan, the TV version has a longer spoken introduction by an uncredited male narrator, with still paintings and drawings used to bring the viewer up to speed on the story. The TV version (which has been released on Japanese LaserDisc and overseas DVDs) has additional footage of the Fremen that lacks the blue color in their eyes, indicating that the scenes were cut before special f/x were added.
The often-rumored 4+ hour "supercut" editions also do not exist. The amount of footage needed for a film almost always exceeds runtime, and the runtime was never meant to exceed 3 hours
Contrary to popular rumors, no 6-hours long director's cut, ever existed. The only "director's cut" of the film was the one shown theatrically; Lynch never had a hand in any other version of Dune. Lynch's original intention was for Dune to have been about 3+ hours long. To that end, about 5 hours was shot. This is also confirmed by author 'Frank Herbert', in the introduction to the book "Eye". It would be impossible for a 6-hour version to exist and even a 5-hour Dune would mean the inclusion of many scenes never intended for the final version (for reasons of redundancy, etc.). It is only necessary to read any of the final scripts for the film to realize that there was never any intention of making Dune more than 4 hours in length at the very most: the script for anything more just was never there.
The most complete edition would have to be the fan-made Michael Warren 2012 version (bootleg). Both the TV version and the theatrical version had scenes not found in the other and Warren basically re-edited them all into what he considered the most complete version he could make from the original source material.