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No need to look further, we have big exchanges with high liquidity so no need to look for more. I personally use binance, bittrex and hitbtc.
I list some though these are common things that we should look up to.
Privacy
Security
Liquidity
24/7 support
Instant deposit and withdrawal
Yeah binance bittrex hitbtc that's (3) three exchange is the most popular as of now and I included poloniex is also one of the most oldest trusted exchange with low transaction fees with strong private keys for the security of the users i am not promoting poloniex or any exchange but base on my own experience i give my own best for those people want to know is the best all about in trading.
Poloniex Era is already over since Circle bought them.I cant really deny about Poloniex service wayback before knowing that this is one of the most popular
but they do dropped down because of such changes.
Back on topic, Security is on of the most important thing for me,next is the transparency,next is the UI, support thing, fast withdrawals and fast response into their support.