Well, there's an all-time, all-time ep.
In terms of the 'fatal' battle, I'm not totally sure that the combat and medical sabotage logic fully held up, but it was a fun fight, gave us some iconic musical fanfare later preserved in Futurama, and three classic Nimoy-Spock moments. One, in which he dejectedly declares himself a criminal in the aftermath. Two, in which his 'human half' temporarily blasts through his cold, Vulcan exterior in a moment of pure joy. And Three, in which he savagely but coldly and logically gets his revenge on his faithless fiancee.
But really there were earlier, amazing moments as well, as with him slowly losing his mind and slipping towards death, and behaving completely inscrutably.
Once again, this ep reminds me that Shatner and Nimoy were some towering iconic, acting figures in TOS, greatly helping to make the series such a smashing, unique breakthrough in SF live media of the time. (also looking at you, Kubrik's "2001")
And not unlike M*A*S*H, which started up a couple years later, the rest of the cast was very strong indeed.
