I think Garak is the most trustable person in Star Trek, that's because he is a liar and he doesn't hide it. He lies even when he doesn't need to, he makes up theses ridiculous lies just because he feels like it, and everyone knows it about him.

That is what makes him so trustable, you can't always trust someone who is honest, you don't know when they might be lying to you, but Garak? You will always know when he is lying because he is always lying, even his truths have lies, and his lies have deeper lies, you know not a word he says to you is true so you know you can always trust him to not be honest and you know what to expect.