Would part of the answer of what we view as the most famous battles be partly dependent on what age group we are talking about?
like i said in a previous post, the composition of the pool surely influence the final result, so characteristics like age, location (and looking to the US i would imagine it depends also from state to state, because in a vast subcontinet like US, i would assume cultural aspects and tendences differ region to region), .. but at the end, in my opinion, as i try to explain before, the factor which go to influence for the most a top 5 chart of this kind, it's the level of passion for this matter of the interviewed, and this can't be brought back to any other feature or specific peculiarity.
Why Thermopilian enters in a top 5 of this kind in the US, much probably is due to the vision of related movies, but surely the same cause would affect this choise everywhere, so also here in Italy, for example. I don't see why Troy, which movie is in an upper class for me, don't cover the same position.
Anyway, don't think history being abandoned just in the US, worldwide people prefer other no conceptual interests or 'much practice' studies. If an historical fact is studied in all its complexity and readed in its several versions we can dispose in that moment in that place, we could even open the mind instead bring it in the conventional interpretation of that fact, which is in the universal optic the only cold comfort we have to provide. This is a reason why history is basically discouraged, and i would say everywhere, in school first.