You can say one sentence and everyone on earth will hear it.

[The Random One]

I would simply relay the message [http://ludonaut.de/the-message/].

 

[Kapri]

"CHEESE FOR EVERYONE!"

Oh, oh! Or...

"I don't know who you are. I don't know what you want. If you are looking for ransom, I can tell you I don't have money. But what I do have are a very particular set of skills; skills I have acquired over a very long career. Skills that make me a nightmare for people like you. If you let my daughter go now, that'll be the end of it. I will not look for you, I will not pursue you. But if you don't, I will look for you, I will find you, and I will kill you."

EDIT: I forgot it was supposed to be under 30 words, whoospy daisy...

 

[Pharsalus]

So much for my dreams, what of yours?

 

[Lashen]

ALL YOUR BASE ARE BELONG TO US​

THIS.

 

[MrHide-Patten]

"MR. POTATO HEAD IS THE ONE TRUE GOD!"

I of course would be shouting.

 

[FrozenCones]

Self Destruct Sequence Activated.

 

[njrk97]

'you guys should check out the escapist'

or

'People of Microsof give rareware back their company or you heads will explode in 3 to 6 days'

 

[StormShaun]

"This is the Archangel of Lightning...people of Earth stop being evil or my space fleet with blast you back to when we created you!"

Then...well I have no idea what will happen afterwards. '