None of these would be a go-to but....Gate.io due to their wide variety of supported coins and low fees. Their trading features and UI are also on par with the larger and more popular exchanges. As far as customer service goes, can't say any of these exchanges has that area covered better than the other. That said, the saying "never keep your funds on an exchange" holds even more true with these guys as the platform isn't the most transparent....
Are you aware that Gate.io used to be Bter? They got 'hacked' for 7000 BTC, declared they would try to earn back the funds through trading fees and then got bored and relaunched like magic.
Even though OKex is a big deal, I find the UI hideous and I don't trust any of these quasi Chinese places to address any problem I might have. I'd wait for my coin to turn up somewhere less shit.