I think the best coin to invest in the long or short term is the USDT coin, because the coin is very stable and very good movement if you buy more, I'm sure that in the future prices will increase again.
Are you kidding ? USDT is a stable-coin and its value will remain always $1.
Bitcoin is always the best for both kind of investment. If you don’t have money to go for altcoins, then you can stick to investing your money in Bitcoin alone. With Bitcoin the price can go up or down at anytime (that’s the risk) if you planned on short term investment, if you’re lucky the price might even go up the next day and you will make profit. Then as for long term, that’s the best thing to do if you can invest at the bottom price and hold it for long term, then be sure to receive a huge amount profit.