I think that stable is a project with small volatility of rate of the token. For example, Bitcoin is rather stable for cryptocurrency.
I got it. Although I do not like the term stable in relation to the project. Apparently, you first of all mean the stability of the asset.
In this case, the most stable assets are coins from the top 10. They have not lost in price more than 50 times (
BCH). On average, losses the best coins are about 90% when dump 2018. No matter how awful it looks, this is a good result 2018 for cryptocurrency

.
To loose 90% from the maximum value is a very bad results. People lost a lot of money during 2018 especially crypto hodlers.