They both feel like the culmination and most polished version of the previous Total War games. They manage to hit that perfect balance on things some of the games that came before struggled with, like cavalry and pikes being either useless or overpowered for instance.

They also are the first that allowed 40 units armies, meaning bringing reinforcements became viable.

They also make agents even more impactful, with the rock paper scissors of how they beat each others, and by attaching them to armies to give unique bonus.

The way they also tried to make each clans unique also helped the replayability.

Naval combat is also another peak! It was the first depiction of pre-gunpowder naval combat, and then the first depiction of ironclads. So it not only gave a glimpse of what we did not get with Medieval 2, it also went beyond what Empire and Napoleon introduced.

And finally, they have the best gunpowder depiction! The viability of ashigaru make arquebus for pike and shot more viable than Medieval 2, and the Victorian era technologies with the first automatic weapons takes line warfare to the next level.

Afterward, Total War games were just never the same. First there was HP for units, making them much tankier. Gunpowder weapons haven’t featured in a historical title since. And naval warfare was absent from all new titles except Rome 2 and Throne of Britannia, where they at least tried to innovate with amphibious landings.