Benzodiazepine is a psychiatric drug that can be used to treat things such as anxiety and insomnia. It can be a sedative, a hypnotic, a muscle relaxant, and more. Different kinds are often abused. As there are many different types formulated for the above variety of reasons, there doesn't seem to be any one "strongest." According to http://www.enotes.com/drugs-substances-encyclopedia/benzodiazepine/overview, some of the stronger ones are alprazolam, lorazepam, triazolam, and clonazepam.