Open CMC top 200 and do a good research, and choose anyone, a lot of coins are good, near, solana, dot, siacoin, eth, matic and a more and more. Just choose 20-30 coins and invest in them, dont hold all money in one altcoin. Its one of powerful rule.
Top 20 is a stretch mate, being conservative on the altcoins is a better way to make money. While it is true that only few of these top 20 will be worth something in the long run, I find it simpler to put all the money in Bitcoin or maximum the top 5 altcoins.
Obviously holding a single altcoin is never recommended. Bitcoin should be the basis of your portfolio and then you should expand on that gradually but be strict about altcoins. Because majority of altcoins will go busy in the long run.