(FOR A STORY!)
Well, do you mean the effect it has on you when you smoke it or the long term effects? If its only when you smoke it, probably tobacco. Because it just makes you feel good for about 30 seconds each time you inhale it. The weakest long term affects, would probably be cannabis, because it doesn't harm your lungs as much. And it stays in your system long, but it doesn't really cause much damage. Besides destroying brain cells, but you only use about 10% of your brain , so destroying a few barin cells won't matter. Yes, i sound like a pothead, but i've never smoked it in my life lol. I just think people make a big deal out of it for nothing.
I'm gonna have to say cannabis.
Cigs or w*ed