As a child of the 80's, I lived through the "Just Say No" campaign, however, due to various substances, I can't remember any of them.

Okay, lame joke.

Not exactly the best, but my favorite one involves a young minority girl talking about the effect of marijuana. She is refuting the claim that marijuana causes memory loss, explaining in mainstreamed ebonics, "well, that's wack, because I've been smoking weed since... since..."

Out of the old school batch, I would have to go with the "This Is Your Brain On Drugs" ad. The one with the egg in a frying pan. Catchy slogan, vivid imagery. A marketing success. It would probably have worked well if they were selling a frying pan, too.

Out of the new school batch, I agree with fugitive247 that Rachel Leigh Cook with a frying pan is very convincing.

Nothing to do with commercials, but the bookmark in my copy of James Elroy's Black Dahlia has "Just Say No!" mock-spraypainted on a brick wall on it.