My all-time favourite:
"It's never easy, but if we turn our backs on our principles we stop being human." - Captain Janeway, "Equinox"
A few others:
"I don't compromise with Borg." - Captain Janeway, "Unimatrix Zero"
"We are what we are and we're doing the best we can. It is not for you to set the standard by which we should be judged." - Captain Picard
"Com par miso, Capitan
. The hall is rented, the orchestra engaged. Now it's time to see if you can dance." - Q, "Q Who?"