Bakemonogatari, 18if Recommendations
I'm actually impressed no one made that recommendation yet.
18if and Bakemonogatari are both divided into arcs, each one following the same pattern : the male protagonist stumbles upon a female character who, because she is confronted to real-life problems, faces supernatural phenomenons. The protagonist then saves said character, helped by a specialist of these phenomenons (in the person of Lily/Meme Oshino). Friendship ensues.
Also, both have particular art styles that make them stand out (that's especially true for Bakemonogatari). The protagonists have pretty much the same personality, same goes for the researcher of 18if and Oshino, and both are mystery/comedy/romance/occult anime.
A guy meets the world of supernatural and starts collecting girls for his harem by helping them solve their problems.
Both have unusual style and are divided into arcs devoted to each of the girls.