"They invade our space, and we fall back. They assimilate entire worlds, and we fall back. Not again. The line must be drawn here! This far, no further!" Jean-Luc Picard, Star Trek: First Contact
That was oddly the first quote that came to my head. Though dozens are now circling.. I'll give you the top three that have stuck with me for various reasons...
"We've grown apart, the lot of us. We didn't mean for it to happen; but it did. The war changed us, pulled us apart. Lisa Cusack was my friend. But you are also my friends, and I want my friends in my life. Because someday we're gonna wake up, and we're gonna find that someone is missing from this circle. And on that day, we're gonna mourn, and we shouldn't have to mourn alone."
-Miles O'Brien, 'The Sound of her Voice'
"It takes courage to look inside yourself, and even more courage to write it for other people to see. I'm proud of you, son"
-Benjamin Sisko, 'Nor the battle to the strong'
"For that one fraction of a second, you were open to options you had never considered. That is the exploration that awaits you. Not mapping stars and studying nebula, but charting the unknown possibilities of existence."
-Q, 'All Good Things'