It depends on your investment objectives and risk tolerance. If you want to just buy and save one cryptourrency, it should be Ethereum. I must warn you rather than holding Ethereum not for the faint of heart. If you're the type that panics sells when you see your investment down 20-50% then it's not for you. If you will quickly take advantage, then it may also not be for you. If you want to invest in the long term and last for at least a few years, Ethereum can be one of the best investments you'll ever make. If you prefer risk, you can use Bitcoin or a combination of Bitcoin and Ethereum. Bitcoin is significantly less volatile but has less upside potential.