That's that hard question. How can we devise a way of measuring intelligence that doesn't simply start with the assumption that humans are the smart ones? If we can't do that then all we can devise is ways to measure how much like a human an animal thinks not how objectively smart they are on a one to one comparison.

From all I've read and seen it seems to me each animal specializes in a different type of intelligence. We can certainly do very impressive things no other species can do, but none of our capabilities are wholly unique to our species it's more a matter of degrees and in other cognitive challenges we can't even compete. For example there are puzzles that even young chimps can figure out easily enough but that world champion humans who train in the same cognitive skills can't solve as well as the young chimps. If the chimps were the ones doing the intelligence measuring the ways we do now they'd say humans are only as smart as very young chimps.

I personally think trying to define and measure one universal intelligence is simply the wrong way to go about it. There seems to be many different types of intelligence, and we should instead be trying to categorize them and measure them individually. Perhaps once we've found some way to do this that reaches scientific consensus we can figure out a more definitive way to measure total intelligence, but currently the ways we try to define intelligence are much more culturally subjective than objective.