The UK government did a scientific survey that ranked drugs in order of their dangers to people and society -- then promptly hid it because (naturally)it showed marijuana far less damaging than tobacco and alcohol and the rankings didn't match up to how drugs were officially classified by the government. Here's the top 10, but go to the article for details on their effects, the number of deaths they cause, number of users in the UK, etc. Find your favorites! P.S. Where's marijuana? #11.
4. Street Methadone
11. Cannabis (Darn, I didn't make the Top Ten!)