AFAIK there are a few that produce such results that they can not be distinguished from a real person. None of the really good ones are open source, and the best ones I have heard of are not even for sale.

The best implementations make an advantage in markets so they are well guarded. We do not know about the best implementations, because we did not notice a thing. For example some phone operators have replaced their customer services with TTS / STT solutions. Because people tend to lock up when they realize they are talking with a computer, they have had to make those systems sound very natural.

I know a few are pretty crappy ones, but a few are plain spooky. Customers that tend to joke and flirt with the computer, hoping for an emotional response, probably are most likely to notice them.

Then there's the case where the US intelligence services demonstrated their capabilities to a politician (senator/congressman) by recording him, and producing a voice clip of him saying something like "death to america" so well that no one could distinguish the speaker. It also seemingly passed further voice analysis. Google it up, pretty interesting read.