Several to mention. The best that come to mind are Nick Naylor's bribe to the Original Marlboro Man, and his final speech on the freedom to choose:
Gentlemen, it's called education. It doesn't come off the side of a cigarette carton. It comes from our teachers, and more importantly our parents. It is the job of every parent to warn their children of all the dangers of the world, including cigarettes, so that one day when they get older they can choose for themselves. I look at my son who was kind enough to come with me today, and I can't help but think that I am responsible for his growth and his development. And I'm proud of that.
Maybe it's just me but Nick answering the senator's question about whether he'd be fine with his own son smoking:
"If he really wants a cigarette, I'll buy him his first pack."
That just...that takes guts there, to say what would probably be an unpopular answer.
This was also treated as a Crowning Moment of Awesome within the film itself, as Nick's emphasis on personal choice was enough to sway public opinion in the cigarette company's favor on that particular issue.
"I'm his father... You're the guy f***ing his Mom."
Turning the Littlest Cancer Patient act onto the anti-smoking people on a talk show, by claiming that the anti-smoking groups want him dead, while the tobacco companies don't want to lose a valuable customer. The Captain himself was very impressed.