i would say poloniex. i remember how they cheated during an xrp pump 3 years ago. they locked the trades for a while and then opened again saying the server is overloaded. anyway nobody uses this exchange anymore.
3 years ago, lots of issues occur with Poloniex, not only with Ripple. The site crashed for so many times, and each time they openned their site, margin crashes killed lots of marginal traders. I remembered those days, and that was one of reasons why Poloniex shut down its troll box.