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?

Try to buy some ethereum because as I observed ethereum right now is one of the most valuable coin among all and theres nothing to lose if you give a try.
I will try to buy tons of medium altcoin rather than ethereum, it looks difficult to the ethereum to go back again to the peak price because it's pegged with the price of bitcoin. Remember so many icos are still doing a lot of pressure to the ethereum. With the latest report about ethereum roadmap 2.0 and it's worth to buy.
Nowadays with so many developments that are experiencing the increase in coins will be very easy to pick a nice coins bought in the near future. like the Ethereum, NEO, and IOTA is able to be the main coin in coin for purchase this time. so it is very good with using it as the main coin in support will be increasingly profitable again.