All the 1977 mixes are authentic-enough original mixes IMO. Although the six-channel mix is a reconstruction, is is very tastefully and expertly done using in-theater recordings as references, and the difference between the original 4.2 mix and the current 5.1 reconstruction should be pretty minor.
“Definitive” really depends on what you’re looking for. If you’re looking for what most 1977 theater-goers heard, that’s the mono mix. If you’re looking for a technology showcase/what the fanciest best-equipped theaters showed, that’s the six-channel mix. The stereo mix is sadly the odd man out – no matter what I consider “definitive”, it never ranks.
In a lot of ways, Star Wars fans fall into two camps – those who want to see the movies exactly as they were in the theater, and those who want to see the movies as if they’d gotten as respectful a Blu-ray release as any other classic film. The former camp tends to favor the 4Kxx releases and the mono mix. The latter camp tends to favor Despecialized and the six-channel mix. Full disclosure: I’m in the latter camp.