Doesn't exist. I believe there are drugs that make people more willing to talk. That doesn't mean what they're gonna say is the truth. If you're talking about a substance that will take someone completely out of their comfort zone, I would say psychedelics would be the best possible way.
I think giving someone enough psychedelics that they become totally frightened you could convince them to do all kinds of crazy shit. I know when I've tripped with my girlfriend in the past when I was still using, as I would start tripping, I would start freaking out over potentially saying the wrong thing.
It's been pretty proven that Barbiturates are not a truth serum. Like I said, they're more likely to talk. That's not the Truth though.