I personally use Gemini for primarily trading Ethereum but dabble with Bittrex and Poloniex (got an account pre-BitLicense).

Poloniex, I don't trust with the majority of my funds for various red-flags that the community has presented and Bittrex doesn't seem to have any retail presence (other than being in an abandoned strip mall in LV).

Gemini is founded by the Winklevi twins.