To this day I did not use in a very intensive manner their support service (I guess I had a message on each of them).
But without any hesitation, I would elect Poloniex as the best of the best :
However Bittrex still has some nice advantages too, such as much more trading tools or more important some very important crypto-currencies missing on Poloniex (Waves as for exemple).
So even though both have advantages over the other one, Poloniex is still the best mainly for its volume.