I just got asked by someone who knows I have lived in all three countries which I think is "the best". Of course, "best" depends on who you are and what you want. Let me say up front that I pick Guatemala but there is more to it than a clear choice for everyone.
The biggest common thing I saw was free health care. All three offered options from free of not wonderful quality to insurance plans and private hospitals. The most "socialized" in Costa Rica where the government owns the insurance company (all insurance) and telecommunications.
Costa Rica
When I escaped from the U.S. that was where I initially lived -- even though I knew much more about Nicaragua due to my involvement in the anti-Contra Aid movement in the 1980s. I ended up in Costa Rica due to my job and it looked like a better place for me than the U.S. I still feel that way even though I have not lived there for 16 years.
I lived in the hills above Alajuela which is in the central valley, not far from San José, the capital. A relatively low stress area with great weather and reasonable prices -- both on food and housing. With little Spanish knowledge, I did OK there. But, I quickly discovered it was too much like the U.S. for me. Generally, a lot of busy, up tight people trying to be Gringos. "Vamos al mall y comer pizza" (Let's go to the mall and eat pizza) is my single sentence to offer up an image.
Now, rural Costa Rica can get very down to earth but usually has weather too hot for me and, at the time, limited Internet access. The beaches are great but visiting the beach is different from living where it is always too hot.
Nicaragua
The biggest plus in Nicaragua are the people. They tend to be open and friendly -- particularly in the rural parts. To contrast the attitude of the people with Ticos, if someone you didn't know passed you on the street carrying a bottle of rum and you said "where's the party" he would probably invite you to come along.
Unlike Costa Rica, the capital, Managua, is way too hot. I ended up in Estelí which is in the mountains. Decent (a little too hot for me) weather and friendly people. To me, the biggest downside was the lack of areas with a cool climate and a government that was "constipated" when you needed to get something from them.
Guatemala
I have lived both in rural Guatemala and in Guatemala City. I don't like cities -- any cities -- and while Guatemala City is no exception, it is a decent city. That is, if you can live in a part that is not more like Gringolandia than Central America. People tend to be friendly with a sense of humor. When Lula jumps on someone, saying "she only eats boys" usually gets a laugh.
There are two things that make Guatemala stand out:
Conclusion
If you are about to make your first move out of your home country, Nicaragua would be hard. While Costa Rica used to have an edge over Guatemala, I expect they are about the same these days with cheaper living and more open people in Guatemala. I would, and have, settled on Guatemala.