For me, it was Gegege no Kitaro. I had been aware of it for a while, but it was just kind of in the back of my mind.
Recently I purchased a set of NOS Lucky Yuckies on ebay, and it kind of ignited my interest in the characters.
I've always loved folklore, and am familiar with Yokai lore, so I always knew Gegege no Kitaro would appeal to me, so I sought out the latest anime iteration, and now I'm hooked.
Now I want to read the original Manga. I need to try to find a source with English translations.