Best TTS engine?

What is the best TTS engine you have heard and like to hear books read with? Cereproc looks very promising and affordable but does it work compatibly with this app?

I personally like the Google voices better than Ivona.  They sound a bit robotic, but the percentage of words that they blatantly mispronounce seems lower.  Haven't tried Cereproc

Google TTS voice for french is good but not perfect.
And the ending of a sentence is always the same, it don't detect ending symbol ? or ! to enphasis the sentence.
 
I use a lot the Cereproc Voice (for less than 2$) and you can add some SSML tag in the editing like pause with <break time="500ms"/> but you can also use 'vocal gesture' (non-speech sound) into synthesis.
There can be found here : https://www.cereproc.com/files/CereVoiceCloudGuide.pdf
 
And there a sample that work on android:
<spurt audio="g0001_004">cough</spurt>, excuse me, <spurt audio="g0001_018">err</spurt>, hello.
 
 
 and there is also some emotion tag like :
Today, <voice emotion='happy'>the sun is shining.</voice>

All those I mention work on the android version and it's offline (for info in take like 320 Mb)
I see nowhere information about android version specific.
 
I personaly try those on my french android version and it's work offline, and can be test on cereproc.com
 
 
And for info those work too
<sig f0='0.5'> your text </sig>
<sig rate='0.5'> your text </sig>
 
BUT it's impossible to use it because @Voice delete the <sig> tag and <spurt> tag, too bad.