Bitcoin.
Because it's value is increasing nowadays and market trading experts say it will cross $30k at end of 2018. It is first decentralized currency and have expectancy to go more valuable in the coming years.
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But if you want to buy any cryptocurrency, go for Ripple. Because Ripple is cheap as compared and has a high expectancy of getting much more valuable in coming years. So if you buy Ripple now, you will have a high amount of money in near future.
Comparing Bitcoin and Ripple or any other currencies no-one can say which can go further because Bitcoin is decentralized and Ripple is centralized which means ripple founders can take all the money and flow (if they want it :P).
Furthermore, the government may ban the cryptocurrencies because there have been several incidents of Bitcoin hacking in last month (the coincheck hack in which hackers stole Bitcoin Worth's $500m) and and a company flew after taking Bitcoins for investment.