bitcoin is the safest bet. altcoins are subject to extreme volatility.
I'd say one of the safest bets, not the only. But there are only 10 cryptocurrencies that are worth serious consideration for investment, of which Bitcoin is one.
Others include:
- Ethereum
- Litecoin
- Ripple
- Monero
- Dash
Here's a list of all cryptocurrencies so you can do your own research:
https://www.cryptocompare.com/coins/#/btc