I enjoyed Detective conan because of it's unique plot. The author does an amazing job coming up with new mysteries to be solved and adding in great characters. The show also has a creative side by giving Conan a unique set of gadgets that he uses to solve the cases that are thrown his way.
This anime has been airing for more than ten successful years and has had 17 movies along with a few specials and OVA's. If you are looking for a long and great mystery anime definitely check out Detective conan.