For investment, I dare directly answer Bitcoin and Ethereum. However, if it is a long-term investment, I do not dare to give a definite answer. I think, Bitcoin (BTC) is one of the few cryptocurrencies that is safe for long term investment. Again, I can't guarantee 100%.
Everyone can't give a 100% guarantee on every cryptocurrency investment, because whatever the investment and from whatever direction everyone will start, it always has risks, so it's very true as you said that there is no 100% guarantee in investment even though cryptocurrency is still very safe to choose.