I still occasionally think about the series on Rome, and also the Rise of the Khans series. (Also, apparently, it's pronounced Jenghis - who knew?!). The series on World War I is also phenomenally interesting and really paints the picture of just how brutal and senseless that war was. The section on Verdun is particularly harrowing.

Another one that I think everyone should hear is "Logical Insanity", where Dan talks through the civilian bombing campaigns of WWII. He makes a case that the nuclear bombing of Hiroshima and Nagasaki fit into this wider pattern of bombings in a really interesting way - those two events obviously loom large in our thinking about history, but in the context of WWII they were almost... mundane. Just two more bombings of civilians in a war that was full of the same. (Dan makes the case in a much more fleshed out way, so please don't take my half-assed description as an indicator of the quality of the episode).

These have all fallen behind the paywall at this point but they are well worth the money and time.

Not sure it counts as a lecture but damn that's a great podcast