xxxHOLiC, Shiki Recommendations
If you like dark/mystery anime like Shiki, you should watch xxxHolic, and the other way around. I couldn't get through with Shiki, but xxxHolic is a lot better. It is simplistic yet surprising, and the story is easy to follow.
All though these are two VERY different anime I find them to have a few similarities like the arts styles are a luttke similar. They can both be dark, although with xxxholic it has more comedy. But for the most part if you like one you should like the other.