The short and best answer for that is definitely Bitcoin.
But I think you're referring to some best altcoins to invest in as we are here on Altcoin Discussion. Well, there are dozens of altcoins right now and some of those coins are new in the market but not all of them have the potential to give you competitive returns for your investment. My honest answer is, Binance Coin (BNB), Ethereum (ETH), Solana (SOL) and Cardano (ADA). These coins that I've stated are also in my portfolio.