Actually, I'm pretty sure nicotine is more addictive. Caffeine's pretty high up there too.
It's also really hard to test the addictiveness of drugs due to the amount of people that are not represented in the tests people due. It is also very subjective, one person may got more addicted to heroin than nicotine, while another person may have the opposite reaction.
There is also the fact that organizations such as the DEA want illegal drugs to be considered more addictive and harmful than legal drugs and tests are often biased.