Intelligence. And I'm speaking in terms of a species as a whole. Go on and choose the outliers and sure, it's not 100%, some mentally handicap may be less intelligence than a bovine.
You don't see cows, chickens, etc. driving around, learning how to read, write, do math, etc. Our capacity to learn and relate experiences with knowledge makes us superior.
Although I would object adamantly, I wouldn't blame aliens if they came and wanted to harvest our bodies or obliterate us and conquer our planet. Survival of the fittest. If they have the means of our destruction and we cannot defend ourselves, then so be it.