Kitchen sink drain replacement
 

This morning I noticed a leak under my sink. We just recently bought the house, so I haven't had time to explore things like this. I found a hybrid system of brass, copper and PVC. I am in the process of replacing all with new to prevent any new issues from popping up, so I am good on that part.
I am struggling with the removal of the sink drain itself. The ring nut is firmly attached. What tool(s) do I need to be using to get this ring nut off so I can continue on with my installation?
Thanks,

I normally use slip-joint adjustable pliers, when you slide them all the way open they're the perfect fit to grab around a kitchen drain pipe.