Amazon Polly [0] and Google's Neural Voices [1] make Watson's voice seem pretty awful by comparison. Unfortunately, they can be costly, depending on what you're doing, and you have to hope neither of those companies cut off your access for arbitrary reasons.

If/when that happens, and you go to look at the rest of the tools... They're pretty crap. Still.

[0] https://aws.amazon.com/polly/

[1] https://cloud.google.com/text-to-speech/