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.
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.