Crypto market is go down in this movement. I have some USDTs. So I want buy some coins. Is this time good for buy? What are the good coins to buy and Hole long term? Any suggestions?

When viewed from a few months back, I think ETH currently has the lowest price, and my advice if you have money for investment, I think you can buy and hold ETH coins.
Probably the safest and more predictable investment is investing in Ethereum. At a minimum, we can expect an increase in price by 2 times. And while the risks are very small.