On the mall in Washington DC, right next to the Washington Monument on 4/20.
Haha, I was in DC selling weed and smoking ALOT at the Hemp Festival and smokeout which was going on a few blocks from the monument. People of all ages smoking weed, I even remember selling to a jamaican guy who brought his 5 yr. old son with him. I had never seen so many hippies in my life. There were 50 yr. olds playing hackysack and stoned as shit it was hilarious. I'm definatly going next year.
Yet, I would have to say... the most public place I ever smoked was in the boy's bathroom of my highschool right next to the lunch room during lunch! Haha, it smelled up the whole bathroom and everyone was talking about it. I got out of there before they sent security in and made everyone in there go to the office.