There are lots of crypto currency nut I think the best one as of today is Bitcoin. In alt coin there's eth that leads followed by Dash, and Monero etc. The reason, I think, behind the popularity of Bitcoin is its big community which resulted for Bitcoin on being the 'best' cryptocurrency today. It might change because it will depend on the market's demand, for instance a new kind of crypto is introduced and more people admired it compared to other cryptos, for the reason that there are lots of cryptocurrency, there's a chance that the leading crypto,which is Bitcoin,  be replaced by a new one.