N7 is implied as the best because humans are implied at being the best, it's just a continuation of the human-centric narrative.
ME 3 never makes a big point about how this one Asari is a Commando, or how this one Salarian is STG, but it will go out of it's way to point out how Shepard is an N7, how Vega is a wannabe N7, and how Kia Leng was an N7 etc. If the human special forces were truly on par with the rest of galaxy the narrative wouldn't make a huge deal about it, but the plot focuses on it, so in the eyes of the story humans are the most special, and our special forces are super special.
Also, the bit about the 'honorary N7s' is a codex entry in the SP, even if one is going to completely discount the MP side of things, we still have a part of the "official" lore going on about how other species are being awarded this title, thus insinuating that the N7s are better than other species' militaries. You don't read anything about humans being awarded 'honorary STG' status, or Asari commandos being inducted as 'honorary Battlemasters' for example.
A narrative, I believe, is fan-interpreted and doesn't actually exist within the framework of the game. From my own personal consumption of the game's content, it was never humanity that was special, but Shepard that was special. As a consequence, humanity received perks and benefits as well as a lot of attention because Shepard was special.
This is a very common syllogistic fallacy that many people misunderstand, and for good reason. For example:
Commander Shepard is special. (Major Premise)
Commander Shepard is human. (Minor Premise)
Thus, all humans are special. (Conclusion)
Just because Commander Shepard is special does not mean, by consequence, all humans are special. If this were the case, all spectres, as an example, would be humans as they are deemed the best the galaxy has to offer. If you actually replay through all three games, you will notice it is exclusively Shepard, and not humanity, that is the driving force of why the game appears to be human-centric, when it's really not about humans at all.
Again, Kai Leng and Vega were merely ways of fleshing out the N7 program because BioWare literally did not explain it, at all, in the first two games. You also can't forget that Kai Leng was a part of Cerberus, a human-only organization, so there are a lot of assumptions and leaps in logic you are making here to come to the conclusion that "N7 is special."
How does an honorary status suggest N7 is somehow superior? A better comparison would be if a mayor is giving someone a key to a city. This person isn't receiving this gift because the mayor is somehow superior and bestowing this honor upon them. The mayor is offering this gift as a means of paying his/her respects to this persons work, dedication, and achievement in society. Being an honorary N7 just means a non-human species did something that N7 program felt was worthy of recognition. As far as why it's never mentioned if STG or another organization has a similar award, perhaps it's just a cultural difference? It certainly doesn't mean, by any stretch of the word, that N7 is better just because they are recognizing achievements of other species.