There are many altcoins are in the market but we can't say any coin's price is very stable for a long period.But some coins price constantly increasing when we analyze them in long terms those are ethereum,litecoin,XMR and DASH.

But these coins price also stable but everyone hoping that these are the coins of future so it is better to invest on these coins than new and cheap coin because these coins are already in the market for long time and got the recognition from investors when we invest on new coins it needs long time to get the recognition from people.