Which Exchange is actually the biggest in moving coins (all together) per day in Dollar? Is there a list somewhere?
the same site you are on has a list of them here:
https://coinmarketcap.com/exchanges/volume/24-hour/polo is on top because it has so many different altcoins and it has so many pumps going on every day. and the value that you see in dollar is actually the converted value to dollar because there is no USD market in 90% of exchanges (polo included)
all of them.
there is no exception, they can all get hacked regardless of their history. but it is higher for less knows and shady exchanges with low volumes.