Android's default TTS (Text to Speech) engine uses Pico engine.
Although I have no problem with it, I think its voice is dry and machine-like.
If you want different voice try Ivona TTS Engine.
There are few other TTS engines available and most of them are paid.
Since Ivona is on beta stage, it provides free service on various languages/voices.
Search for Ivona from Google Play. You will see main engine and different voices.
You must install Ivona Text to Speech Engint HQ first.
Download langagues you want to install.
Feel free to download different voices and test them out.
After installing HQ engine and voice engine,you will have to download actual voice data.
The voice data size is around 220mb - 250mb depends on the language.
Once voice data download is finished, you will be able to test out.
Test out different voices and find out your preference.
I chose British accent Amy one :)
Now you will have to change the TTS setting to Ivona.
Go to the setting and find out Voice input & output option.
Check enable Ivona Text-to-Speech HQ
Then change Default Engine from Pico to Ivona.
Go to Language option and change it to your downloaded voice.
Hit Listen to an Example to test out.
If a certain app is not using the new voice,
check Always use my setting check box.
Now enjoy the new voice of your phone :)
I found the Ivona apps are running constantly.
If you don't want unused voice engine running, you may uninstall them completely.
Although you delete out the engine apps,
the voice data (200mb+) will be still remained in your phone.