Since it’s been 31 years, I think we can confidently reveal that Spock sacrifices his life at the end of Star Trek II.  I wish I could have found a longer cut of it, but this picks up after Spock has stabilized the engine core and exposed himself to lethal amounts of radiation.  This picks up just after Kirk arrives down in the engine room to find his best friend fading.  It’s one of the greatest screen deaths of all-time (somewhat mitigated by the fact that it didn’t stick) and when you take away all the science fiction trappings and settings, it’s really just two best friends saying goodbye.  It’s the most powerful scene in the entire Star Trek franchise, and given the amount of material there is to draw from, that’s saying quite a lot.