Your given a microphone while standing before every person alive. Whatever you say will be heard and understood by all. You have undivided attention and can say only one sentence. What do you say?
A very long run on sentence:
Don’t worry everyone, be happy because all this pain is an illusion, there is no hell; smoke a joint please.
Something along those lines. 
I can agree with that.
“Some men see things as they are and say ‘Why?’ I dream things that never were and say, ‘Why not?’”
This quote by Ted Kennedy during the burial of his brother is probably one of the most influential sentences I’ve heard in my life thus far.
Stop shooting each other and start shooting the ones that are telling you to shoot each other.
Excuse me, does any of you have any grey poupon?
Nice.
‘If you don’t croon the ubermensch, you’re the respondent.’ 
Actually, this would be better:
‘God hates rats.’
“Humanity is a plague and a blight upon manifest existence, please do the virtuous and righteous thing, kill the person next to you, and if you happen to be standing alone, do us all a favor, and cap yourself; continue this process for three days, after which, if you are still living, you can continue.”
Let the cleansing begin.
"We’re all goin’ to Heaven, Lads! Waaahhheeyyyy!!
-BOOM-
“I am God.”
Hopefully enough people will believe it that I will have a nice little following. It would make my life easier.
Until they shoot you 
It’s okay if you all die, just make something better than you survive you.
(beautiful avatar, apaosha)
SHOP AT HILLBILLY PETES!
lo no seriously i would say this.
“hello all you retards, now listen and listen good! stfu about all this nonsense and stop fucking fighting eachothre or i will come to your house and fuck you up… hardcore style”
then i would say HI MOM!
No one knows.
Is that sarcasm? It’s not supposed to be beautiful, it’s supposed to be scary …
Rawr!
etc. 
Probably:
“No one saves us but ourselves. No one can and no one may. We ourselves must walk the path.”
Or maybe:
“Have compassion for all beings, rich and poor alike; each has their suffering. Some suffer too much, others too little.”
I find effective horror pictures beautiful. I think it’s all about the “punisher” kind of impression.
I would deliver a revised version of John Galt’s speech.