"Who are you?"
By multiple reports, those will be the first words of The Last Jedi. (Yes, there are some disputing reports too).
But what does it mean?
The most common thought is that Luke genuinely doesn’t know who Rey is.
Another thought that came to me is that he does know, but he’s asking if she knows herself.
And the third possibility? That we don’t see the earbuds for his JediPod, and he was up on that cliff top enjoying some tunes, and his first words are actually:
“Whoooooo are you… Oo-ooo, Oo-ooo…”
"Don't call me a dirty old man.... Cuz I'm not old... I'm of a fine vintage..."
"Get your mind out the gutter... you're crowding me."
Another smiley everyone can use: