Well, what are you looking for? Just cheap living or can you spend some money? Do you want beach activities? Is easy access to the US (flights or driving?) important? Do you want to live near other Americans? Big city or quiet country life? Do you want lots of strip clubs and brothels, opportunities to meet civilian girls, or something else? Do you speak Spanish?
International Living is a good site to learn about where to live as an expat, and they have this page:
https://internationalliving.com/best...ico-u-s-expat/
It's not at all monger oriented, but has some good advice.
Personally, I'm shooting for Ensenada. I like the closeness to Tijuana and the US border, the easy access to the ocean, and the number of Americans in the area. It's a bit pricier than some places but not the most expensive.