I believe Poloniex has the most volume by a long shot compared to other exchanges. There is also shapeshift...which is a type of exchange. Not sure about shapeshift's volume though.
Largest XMR exchanges?
I've been looking around but not certain I have identified the largest XMR/BTX exchange by volume. Seems to be Poloniex. Is there a more busy exchange for Monero/XMR besides that one?