What is the best voice changer on windows pc discord gaming?

I would say in terms of quality, utility and modularity it's got to be Echo Live (voicechanger.live) 

It has a full suite of DSP effects and voices + DSP voice lab that allows you to configure and change the voices or make new effects with complete control

It's compatible with two different types of open source AI voice frameworks in RVC and Beatrice - so you have both high quality voices as well as fast CPU based voices. It allows you to combine AI voices and DSP voices so you can e.g. add atmospheric effects on top of AI voices, or use AI voices to establish a pitch range, that you then layer DSP on top of. 

It has fully functional soundboard that works with hotkeys and also allows you to map hotkeys to groups of sounds - so you can e.g. map hotkeys to a situation and trigger random sounds. 

Also supports TTS and has Chrome extension that works on chromebooks.

If you are only interested in pure high quality AI voice changing and don't mind working with Python - Vonovox or Applio's Realtime are theoretically viable free options - but in terms of all around modularity and ease of use Echo is best imo