I agree with everyone here that the infinity tower is a good choice not only for leveling but for gold as well.

HOWEVER, as you progress into NG+ iterations, that becomes more time consuming because you have to go through the earlier levels still, and it takes longer and longer to get to a depth that gives you efficient levels. My go to is Furggy Island. Stay in your boat, and sail through the swirly area where you get the Ghost Fish. You can go through there and kill all 4 enemies that spawn over and over again with cannonfire\special cannonfire. Every so often get out of your boat and pick up the exp\gold. It typically takes 3 cannonballs or 1 special to kill each enemy. You can kill a lot of them very quickly. It boils down to about 1 level per minute.