I think it has to be this one (nearly 60 words!) from Commissioner Bele to Kirk and Spock: Yes, he will evade, delay, and escape again and in the process put thousands of innocent beings at each other's throats, getting them to kill and maim for a cause which they have no stake in but which he will force them to violently espouse by twisting their minds with his lies, his loathsome accusations and his foul threats. What a great casting choice. I can't imagine anyone but Frank Gorshin delivering that line.
Actors love it when their character gets to make a speech. In contrast to George Takei always pining for Sulu to command, Walter Koenig once said he wouldn't care if Chekov got busted down to nothing and put in a jail cell, as long as he got a three minute soliloquy. And at the opposite end, Leonard Nimoy said the hardest scene for him to play was having to stand there with no lines and nothing to do.
59 words, to be exact -- or 7 words longer than the Preamble to the U.S. Constitution, which must be the second longest sentence spoken in Trek TOS/TAS.