You need to create a big list of contractors and weed them out. Start the list from:
- Word of mouth recommendations
- Google Reviews
- Angie’s List
- Yelp
- BBB.org
- Home Advisor
They're not all good contractors here though. Word of mouth is the way to go. Chances are your friend or your neighbor or someone of your relatives recently had a similar problem. Just ask people.
As someone said in this topic, Rinnai is a good brand. Rheem and Navien are great too. We chose Rinnai for our place from this great guide on how to choose the best tankless water heater.
They review propane and electric tankless water heaters too. Check them out.