If someone only wants to invest in a single crypto, it should be Bitcoin. However, I think this is also not recommended. Instead, diversification is necessary because every investment carries risk, without exception.
There is no 100% guarantee that bitcoin will recover and increase in price in the future, or that the bear market will last longer than expected. So diversification remains the better approach. However, diversification does not mean buying multiple altcoin. Diversification truly means allocating capital across different asset classes instead of concentrating your entire portfolio in crypto.
Even as a single Bitcoin it's not advisable, it's better you invest on other investments and spread it so you don't have to depend on Bitcoin to give you some return. Although, some altcoins are not as that worse compare to other coins, I can put my money on sol, Ethereum and BNB comfortably without feeling any remorse, this will give me some confidence because over the years they have proven to be one of the best altcoins in the market that survived.
I will always say this for people that loves meme coins, I can see the recent trend in what's going around the market, I don't like them and it seems the money is moving back to them. I don't know why people don't learn from history but those coins are not good investment. I don't care if people are doing it for any fun like they said, it's not a good thing to ever try.