Ask HN: What is the most advanced text-to-speech tool/api available?

SABLE is pretty advanced. It is a "XML-based [language that] allows users to add addition controlling commands in text to affect the output."

https://www.cs.cmu.edu/~awb/festival_demos/sable.html

It's part of the Festival TTS system.

Amazon's Polly?

https://aws.amazon.com/polly/