For the long term, of course investing in crypto which is currently top 20 is profitable and safe, but for the short term, investing in coins that are still cheap or below $ 0.5 is what will make our money skyrocket.
There is no such a thing as a safe top 20 coins. If we look at all the fundamentals and ignore all our personal opinion on specific coins, then it's just BTC, ETH, XRP, BCH, EOS, LTC, XMR to be the safest options to go for. All other options are too much of a gamble and have no long term viability in my opinion. Altcoins tend to replace each other quite regularly, and this will continue to happen. I personally only see value in Bitcoin but people are different and for that reason have different preferences. Just make sure you don't expose yourself to a lot of risk by having large altcoin positions. Bitcoin is for the stability and altcoins can give you an extra boost as for the speculative side.