Ah the best loot. Great question. And for that, there is no answer. Only another question: will my fully upgraded (for the sake of argument) superior weapon I found while scouring the ruins of Old New York be better than your brand spanking new exotic you found in Charlemagne's vault on Mars? (btw, the answer is probably yes) Why? Investment, my dear Watson. Your new Thunderlord may have a lot of fancy bells and whistles but my fully upgraded Pulse Rifle may having homing bullets or some other intense ability that we can't even imagine. It all comes down to how you use it. Long story short, there won't be a single location that has the best loot. Yes there will be certain areas with better loot; but for all you know, you missed an exotic in a cave on your way to a mission that rewards you with a common. It's all relative.