I think the altcoin that best fits your criteria is Monero.  It is a solid privacy coin with a firm toehold in the top ten of all cryptocurrencies, and has a long track record of outperforming Bitcoin with remarkable consistency.  It is THE leading privacy coin by a wide margin, and can reasonably be called the gold standard in privacy coins.  The fact that it is a privacy coin, is an important distinction: As governments impose ever more onerous restrictions on the flow of capital and private transactions, privacy coins will experience a major surge in popularity.  No less of a legend than John McAfee agrees: https://twitter.com/officialmcafee/status/941003398215761922

If you wanted life changing gains on a relatively small investment, there are other coins I could recommend.  But, for slow and steady, relatively safe gains, XMR would be my top pick, hands down.