In my opinion, there will never have such a thing as the most reliable altcoin in this crypto world cause its volatile and unpredictable and its not only happen in this crypto world, you can find them in commodity, forex, index.
But if you want to buy a good one, I suggest you buy the top rank cryptocurrency and they already has their use case like ETH, NEO, Tron, Vechain. But you can do your own research too. after you do your research you can decide it ^^