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? 
It is very difficult to say right now as to which coin is best to buy. Prices are still bleeding, BTC is quite stable.
Keep analyzing the marketplace and when the tide starts to turn, get in there. Very difficult to predict at the moment. For me the most promising long term investment is Electroneum. They have a great mobile miner and instant payment system which should help the price improve over the next 12 months.
For me, LTC, BTC, Ethereum...great time to buy.