Ayakashi Triangle

Gender-bender comedy with action. Ayakashi is just a bit more ecchi than Ranma. 

both animes have characters fighting evil spirits 

a bit of ecchi ever then and there, funny mcs 

GOOD SHIT, GREAT FANSERVICE AND FIGHT SCENES, COMEDY/ECCHI STUFF 

They both star two close friends, one of them gets gender flipped, and there's a lot of humor in which the other starts questioning their feelings for the one gender flipped. 

Both feature battles with supernatural creatures and strike a good balance between the action and fanservice elements 

Both are ecchi series with a plot revolving around protecting humans from spirits and the supernatural. In both, one MC is a powerful exorcist and another MC is has an abundance of spiritual energy that they use to support their partner is exorcising yokai. Ayakashi Triangle leans a bit more comedy, while Isuca is a tad more serious, but that does not get in the way of classic ecchi shenanigans that are abundant in both series. 

Natsume Yuujinchou has the same thematic setting as Ayakashi Triangle; normal human world that has yokai/ayakashi that only a select few can see. Both has conflicts arising between the yokai and human worlds that are handled differently. In Ayakashi Triangle, this typically is solved by eradicating Yokai, while in Natsume Yuujinchou it is more so solved by negotiation and dialogue. So if you liked the setting in Ayakashi Triangle, but wanted a slower but more peaceful way of handling the Yokai-human conflicts (and wanted to skip the romance parts of Ayakashi triangle), then Natsume Yuujinchou might be the show for you. Both also has a powerful yokai cat-like creature as a sidekick that are helpful to the main characters but both has ulterior motives. read more 

Both of these series have action, supernatural creatures, romance, and (slight spoiler I guess?) a heroine that is part human and part something supernatural; seen as a god and are targeted by the supernatural creatures because of this. Ayakashi Triangle focuses on Ayakashi in general, where as Mononogatari is focused on tsukumogami/marebito. The protagonists are also prejudiced against said supernatural creatures and dislike them strongly at first, the heroines however are okay with them. 

Same artist both are very ecchi focused.