Purely theoretically, a coin that is worth almost nothing (like 1 satoshi or fractions of satoshi) has the highest potential for big ROI if it suddenly explodes. Just like Bitcoin went from costing a few cents to thousands of dollars. But on practice it's almost never happens, and if you try to chase such investment, you will end up broke. Trading isn't about making crazy returns on a single bet, it's about steadily making profit over longer periods of time.
Even now, there might be few coins which are launched at the minimum price but once they hit the exchanges they suddenly bloom up giving highest ROI. OP can try such coins if he is capable to manage the risk and yet invest in such coins. Any of such newly launched coins might even have a dump considering the bounty hunters, future of the project, etc.
We can only get the highest ROI from such new coins I guess because it is a bit difficult for the existing coins to suddenly have some pump. New coins usually have lower price at the initial stage but the price rises up suddenly once there is any strong news from the development team.