Actually, this question makes me a bit angry (no, no, not at the questioner) because when I've been researching stuff, and I keep finding references to things that aren't there anymore.
And that's sad, to know that there are places in the world that used to be astoundingly beautiful that aren't anymore, because people were insane, or stupid.
So, without further ado, I present my "I'm glad your empire's not around anymore, because if you used the Parthenon for a gunpowder depot, you fail the cosmic Darwin test" awards.*
1. Baghdad. Mongols. I don't really have the stomach for details.
2. Imagine what Beijing would be like if the English and French hadn't destroyed the old Summer Palace, and if Mao hadn't ruined the old historical center of the city to fit a million people in Tienanmen Square. And seriously, either the UK and France should pay to rebuild the Summer Palace, or the People's Liberation Army should get a week to do whatever redecorating at Versailles and Windsor Castle it thinks would be fair under the circumstances.
3. Warsaw, which after the 1945 Uprising had 85% of the buildings leveled by the Nazis, who used flamethrowers, and who took out the stuff with the greatest cultural resonance for the Polish people first. What they did to the people in the city was of course worse, but I'll not rant about that here.
Of course, there are other examples, but I find it interesting that some of the cities that might figure in such a list like Moscow, Dresden, Berlin and Wurzburg have aggressively been rebuilding what they lost in prior decades, which I am actually heartily in favor of because these cities accumulated over time around existing spaces, structures, narratives and images, and the loss of these monuments--whether in Allied WWII bombing or to clear space for some horrible Palace of Soviets--wounds the city itself in a very real sense.
And of course destroyed cities further afield from present day like Tenochtitlan and Carthage would be very cool too, although perhaps because their destructions occurred further back they somehow don't seem as egregious.
Sorry for not keeping with the desired form!
*Of course the Venetians actually fired on said gunpowder depot, which is actually probably worse, but hey they don't have an empire any more either, so well nyah to them too.