“Peccavi.”
(Not our @peccavi, obviously. But an iconic bit in History that appeals to the Classical training in me.
Attributed to General Napier, upon taking Sind (India) against orders (the Latin translation of peccavi is “I have sinned”), but, unfortunately:
Wikipedia
The true author of the pun was, however, Englishwoman Catherine Winkworth, who submitted it to Punch, which then printed it as a factual report. Later, Napier made several comments on the Sindh adventure to the effect of: “If this was a piece of rascality, it was a noble piece of rascality!”
In adopting this as my SDMB name, I have subscribed to the philosophy of Maxwell Scott:
Ransom Stoddard: You’re not going to use the story, Mr. Scott?
Maxwell Scott: No, sir. This is the West, sir. When the legend becomes fact, print the legend.
I seem to remember reading somewhere that Napier didn’t actually send such a message, but rather it was a suggestion by a schoolgirl to a newspaper (?Punch magazine) that he should have said it.
I think I’ve heard that myself recently. Maybe just 2 post ago. 
Ahem. I was double-checking my memory.
“J’Accuse…!”
ETA: I hope that doesn’t need a translation.
“Hold my beer”
Tora Tora Tora!
“Not a step back!” also known “Not one step backwards”
“It’s a free concert from now on!”
“There are no words.”
“It’s outta there!!! It’s gone!!! It’s 715!!!”
“We find for the defendant, Orenjal…Orenthal James Simpson, not guilty of the crime of murder…”
“beep”
“La Garde meurt et ne se rend pas!”
altough Cambrone said in later interviews he didn’t said it, since he surrendered and didn’t die…
but his following famous word was indeed said…
C’est magnifique, mais ce n’est pas la guerre: c’est de la folie
(“It is magnificent, but it is not war: it is madness”).
50 years and one day ago…
“Iceberg, Right Ahead!”
Yup, watched it again today and choked up again. At least this time wasn’t like watching it live on TV, when I leaped from my chair and screamed “RUN! RUN! RUN!” all through the stretch drive.
“Okay Houston, we’ve had a problem here.”
“Now I am become Death, the destroyer of worlds”
“Fourscore and seven years ago,… .”
“I have to tell you now that no such undertaking has been received”
“The lamps are going out all over Europe, we shall not see them lit again in our life-time”
"Roll up that map; it will not be wanted these ten years. "
“There seems to be something wrong with our bloody ships today.”
“You will come home before the leaves have fallen from the trees.”
“I do not consider Hitler to be as bad as he is depicted. He is showing an ability which is amazing and he seems to be gaining his victories without much bloodshed.”