Destruction Princess

This manga had a concept that I would really like to see used again, something to the extent of a childhood friend/romantic interest has becoming a cruel villain and the main character having to deal with it. This manga does a pretty good job with the set-up, though ultimately it doesn't really use it much for plot purposes, wherein ultimately the plot doesn't seem to matter much beyond just being a setup. Rather, the manga is focused pretty much entirely on comedy, with everything being exaggerated and utterly ridiculous. Still, I did like the dynamic between a normal guy and evil empress, and the various side characters were decent enough to spice things up, so the story was still decent. The comedy was really great though, with the manga being amusing from beginning to end in a lot of ridiculous and immensely strange ways, and to a degree strong enough to hold up the manga entirely on its own. The art was also pretty good. 

tl;dr: A hilarious comedy revolving around a pretty unique romance.