It depends on how much DIY you want to do. Snips.ai is the easiest to set up and can be installed from the Hass.io dashboard.

There is also Rhasspy[1], a voice assistant toolkit for Home Assitsant. It has instructions and custom components on how to integrate a full voice assistant (wake up, speech to text, intent recognition, text to speech) into Home Assistant using solely open source tools with different choices per category. It's very cool.

[1]: https://github.com/synesthesiam/rhasspy-assistant