I chose FOREVER's ending for
the ending, plus the acting, the tragedy of course, plus the message which foreshadows Spock's death in TREK II of one life for many. Morning laziness kept me from elaborating too much, but it's an obvious candidate for most classic moment for all the obvious reasons. Even edited from Harlan Ellison's original version it still had a helluva lot of power. We'll never know whether it would have been received as well had Kirk deliberately prepared to SAVE Edith as in the original script. But that's what true love does to you.