Bitcoin is the most reliable cryptocurrency for long term investment. Since it isn't a pump and dump coin and it has an actual utility, you can always believe in its long term sustainability, though i'd not advise people to expect their money to double/triple, of course it can obviously happen, but that could make them expect too much and prolly fall under pressure in the short term if Bitcoin isn't do well at that time. Having said that, remember that diversification isn't investing in two cryptocurrencies, to diversify you'd have to invest in two assets that are intrinsically dissimilar.