There are so many undervalued coins in the crypto market, but since you can't invest them all here's my suggestion, you already know about Polkadot right, a cross chain with less fees and faster than Ethereum, it's seems like Polkadot is certainly going to be the next Ethereum, during last year if I remember clearly $DOT was just about $5, but it managed to grow during this bull run it increased to $37+ and to be honest it's still a low price for a coin that so many feel it will over take eth, so investing a few dollars and owning even 1 $DOT now won't hurt, it will be worth more in the future no doubt.