Frankly, I would be sceptical of anything other than highly specific comparisons. There are simply too many differences in the situation and too many factors in question to make any kind of valid conclusion by comparing the Aztec to another civilization or empire in a broad way. So, I would question the validity of comparing the Aztecs to, say, the Achaeminids (to use an example raised upthread) in general. Now, if you were comparing Aztec statecraft or diplomatic practices to the statecraft of the Achaeminid Empire, you'd be on firmer ground. You'd still have to make the argument for why it is valid to make the comparison, and why it leads to a meaningful conclusion, but you could at least attempt to do so with success as a genuine possibility. Additionally, I think any question of how impressive the Aztecs were, given their situation and limitations, compared to another empire or civilization with a different situation and different limitations is too dependent of personal opinion and the vagueries of individual variation to have a real historical answer.