Still ethereum is the best choice for investment. Price is near hand and you can cut a big profit here if you invest on it. Besides, Ripple, Cardano, Stellar are the good choice also. Don't buy dead coin by getting low price. Hopefully your investment will give you good feedback.
For some people, 100 dollars may seem a huge sum, while the others invest in crypto thousands and do not care about hundreds. I think Cardano and TRON are worth investing in them much money now.
100 dollars per month is quite big for me but it's a good amount for investment, especially for a long term investment.
Crypto market is really hot right now, if you see it, the price of all crypto is increasing. For which one is good to invest, i guess bitcoin is the best one, because bitcoin price is increasing since yesterday.