"1) The Death of Spock (WOK)
No single character anchored Star Trek; two did. Almost from the very beginning, the chemistry between Kirk and Spock defined the show in a way that made it impossible to have one without the other.
“I designed Kirk and Spock to complete each other— And, in fact, the Kirk/Spock/McCoy triad proved to be the dramatic embodiment of the parts of one person: logic, emotion, and the balance between them,” Roddenberry said.
It was this interdependency that made the death of Spock arguably the most unexpected, dramatic and emotional sequence in the entire franchise. The protective screen dividing them becomes a metaphor for not only the lines between life and death and human and Vulcan, but also for a heart broken in two. As Kirk slumps to the floor next to his fallen comrade, he too is dying."

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Star magazine’s Into Darkness special, “The Top 100 moments of Star Trek” (Emphasis mine)