Bitcoins will always be the number one answer for this question but if you are pertaining the good coins to buy today rather than bitcoins i will highly recommend the ethereum because they are taking the top 2 spot in the market that has the most amount or value in the market. And a lot of articles are saying that they are gonna be the next bitcoins and their value will also pump just like what happen to bitcoins before when they are just getting started. That ethereum of yours will gonna make you a millionaire.
I don't think so bitcoin will give us good profit in short time, i accept Bitcoin is number one but if you are looking to make money in short period choose good altcoins. ETH will definitely make you a millionaire, but here also you need to wait. My best coin for today is Cardano I invested when it was $0.1 now it increased $0.5. I am not telling to invest in this coin I am just saying to find this kind of coin and make money in short time.