The most stable altcoin to be proven so far is Ethereum for sure. In the last few months where the entire market including bitcoin was red ethereum held its own and was the stable of all. Its percentage of deviation was least of any in the top ten coins. It still is going strong trading around 750 usd. Only 250 awaay from its high of 1000. While bitcoin is around 9000 usd from an high of 20000 usd.