What are the most promising low market cap coins that are still under $1?
It doesn't really matter if a coin is under $1, because you must consider also the total supply of the coin. If a coin supply is 100 trillion, it will be under $1 even if its market capitalization is higher than Bitcoin's. You should be looking at market cap instead. It's still not a perfect measure of the coin's value, but it's as close as you get.
Looking for it only may not be the best thing to do for better analyze it more then check the everything that it can be for so you must be more productive for you to have what are really wanted to take for study not just once only but all the possible that it can be.