WN: Maou-gun Saikyou no Majutsushi wa Ningen Datta
This particular WN is remarkable for a lot of reasons. The protagonist of this story is reincarnated in another world as the adopted son of a lich general in the service of Dairokuten Maou (lol, if you can guess who else got reincarnated, I'll clap in congrats), and he joins the demonic armies as a commander while hiding the fact that he is human behind a skull mask and lich's robes. The story itself begins after he has already begun to become famous as a conquering commander but before his real talents have been shown to the world. The protagonist really is the most powerful mage in the story, having survived his grandpa's spartan (and inhuman) training methods, but his major talents actually lie in strategy and administration, meaning he is more general and administrator than he is a mage most of the time.
There is no actual romance in the main story (despite the fact that about seven women have fallen for him by the end), so those who desire romance in their stories shouldn't go after this one. The closest he comes to a romance is his surprisingly intimate relationship with Sati, his maid who was a slave of the lord of a city he conquered in the prologue. Within the story, Sati is the personification of the 'common humankind', whereas Jiron (the protagonist's aide-de-campe, who is an orc) represents the monster races' common people. The viewpoints these two provide during the story just with their comments and actions help to provide perspective, a technique that is often used by skilled writers.
Most of the story goes back and forth between Ike (the protagonist) overcoming incredible odds to push the Demon King's agenda forward and him building up his territory of Ivalice using his knowledge of Earth. I honestly thought a lot that him revealing his true self to Lilith would have made for interesting results, but, unfortunately, to the very end, Ike only reveals himself to a very select group of people who saw through him anyway.
This story is complete, thankfully, with a prequel gaiden story and a single after story attached after. If you like strategist/tactician protagonists in fantasy worlds, it is a good choice to read.