As of now, no one's altcoins are stable the market seems very volatile this year and probably, not the best so far but again i could say for long term stability i could say ethereum has been good and quite stable for its rank as second to bitcoin.
Weak market conditions make many altcoins unstable, including ethereum, which for some time tends to decrease its stability. However, ethereum is a strong altcoin and I think it will be able to recover.