To all the people replying humans, I say bullcrap. I'm not saying that humans are by any means angels, but come on, they are not the bottom of the barrel.
there are more untrustworthy races in star trek. How about the romulans (lying every other appearance),yridians(has there ever been a yridian who wasn't a liar on star trek?), pakled(their entire progress is based off of deception for crying out loud), ferengi(rip off artists)... Hell, half of the androids in TNG we've seen have been lying sociopaths (lore). there are more but why bother? Humans are obviously not the most untrustworthy species.
few are more untrustworthy than the pakleds.
From memory alpha:
The Pakleds were a heavyset humanoid species native to the Alpha Quadrant. To the casual observer the Pakleds appear to be intellectually challenged, especially in their verbal skills. However, despite the marked lack of intellect they proved to be far more cunning than they appeared, and used their innocuous appearance to deceive other races.
is this a one dimensional boring trek race? yeah, sure it is. But they are a valid star trek race, and from what we've seen, they are far more untrustworthy than humans.
Not necessarily, no. See, with humans, you don't know what you're getting. With Pakleds, Romulans, Borg, Klingons, etc., you can bet that treachery may be a serious part of their culture, but with humans? Hell, no. We'll smile, shake your hand, share our noble sentiments, invite you into our homes, make you a part of the family, and all the while we could be taking everything you hold dear, or preparing to end your life. No one is more treacherous than a human, we're just very good at hiding it.