I am not try to FOMO kucoin, but they are responsive than polo and bittrex. Bittrex new UI is great, but still too many junk coins listed. I didn't use Polo that much anymore so no comment.
I felt Kucoin listing more quality coin than Bittrex, especially you are interested in Chinese related altcoins. Delighted to see its volume is increasing now. Hope to see more exchanges coming up with better UI, liquidity, customer service.