The lowest supply coin with the most undervalued price right now I would say is probably Omni. Only 560,000 coins in circulation, and the price is barely above $35 apiece, which is absolutely crazy since Tether and Maidsafecoin are both hosted on the Omni layer.

Another one would be ZCash, it was hyped up really early on but now the hype died down, so prices are extremely low. Less than 4 million ZCash in circulation, could be worth a buy (though the fact that they essentially tax miners is repelling, at least for now).

But that doesn't mean either is the best available investment option, not all coins with low supply are good investments. The best investment I can think of currently is probably IOTA.