I think that if you want to invest in one cryptocurrency it is only Bitcoin. Because bitcoin has halving every 4 years and the number of total mined bitcoins discreases. The total issue of bitcoin is 21 mln BTC and it is limited. It means that the price of Bitcoin will grow over the time.
I think This should be read from OP's post "
BTC and Eth top my list as one of the more secure coins, but other altcoins have proven to be reliable overtime, if you agree. Despite market volatility and the possibility of a crash, which is your most valid choice right now"
That means, OP already has his Featured Coin like you said just asking us what has the most potential besides that. But, it's okay. What you say is true and the ultimate goal of talking to BTC is also because BTC has advantages that other coins/tokens don't have.