What's wrong with Bitmex? I've recently started trading there and I'm a big fan so far. Pretty thick order books, advanced order types, responsive support. I'm impressed so far.

What are you looking for, exactly? Poloniex operates out of the US and serves US customers without a license, and they only offer USDT (risk exposure to Bitfinex). Bitfinex is apparently starting to have a support backlog resembling Poloniex now, and they have been blacklisted by the fiat banking system since early this year (not a good sign if you ask me). Plus, the margin available on Bitfinex is pretty limited. There's Kraken, but it has terrible, glitchy trade engine.

The competition seemed to come down to Okcoin futures and Bitmex. Bitmex has better liquidity... and they just feel less scammy to me. I miss 1Broker's BTCUSD market; they are supposedly launching 1Fox to replace it by the end of Q3....