There are a lot of ways and saying this is the "best" would be highly subjective.
But I could say that champion spawns would be the fastest way to make a lot of gold fast. Baraccoon would be the fastest. (Considering you are not raided)
I suggest doing this with 3-6 people.
You get about 250k-500k gold depending on where you make the killing blow on a champion. Try to lure the champion in an open place with minimum water, wall, impassable terrain. This way your champions gold drop will be maximized.
Then use auto looters, horde minions, giant beetles to collect the gold and transfer them to your bank.
This way you can make up to 500k every 20 minutes, depending on your gear. Which means 500 gold/hour if you are 3 people with good gear. Plus you get power scrolls, champion artifacts and some loot. Which may more likely be worth much more than the cash you are getting.