Thaddeus Stevens explaining how it is that he can say that some men may possibly be inferior to others. After being forced by political necessity to disavow his strong belief that black people should be considered completely equal, and instead say that they should only be considered equal before the law, Democrats Pendleton and Wood try to goad him into stating his true opinion. Stevens, after repeating his line like a Survival Mantra a few times, finally looks at the Democrats and uses Pendleton as a prime example of his argument—because someone "more reptile than man" and with "cold, pallid slime" for blood, is so clearly unequal to his fellow men, yet he is entitled to equal treatment before the law.
George Yeaman changing his mind and voting for the amendment. Loudly.
The Congressmen who voted for the Amendment's passage breaking into "The Battle Cry of Freedom."
When Congressman Clay Hawkins, who up until that time had seemed like nothing more than a corrupt coward, first accepting Lincoln and Bilboe's bribe in exchange for his vote, then switching back for fear of what the people in his hometown will do to him if they find out that he voted for the amendment, finally screws his courage to the sticking place and votes for the amendment, loudly proclaiming that anyone who doesn't like it can "Shoot me dead, I am voting yes!"
Which was quickly followed by another Sudden Principled Stand using the same words, which was quickly hedged into an abstention, rather than a vote in favor.
When the Speaker of the House, Schuyler Colfax, decides to cast his vote for the amendment at the very end, despite not needing to. As everyone waits to hear his vote, he says "Aye, of course." as if it was the most obvious thing in the world.
At this point in history it was customary for the Speaker of the House not to vote on legislation, but simply maintain order and direct the business of the House. Of course, there was nothing that said he couldn't vote.
Lincoln at the telegraph office, mulling over his bringing the Confederate peace delegates into Washington in order to mollify the Republican moderates (although it would kill political support for the Amendment). He openly talks to the two telegraph operators about the whole concept of the phrase "self-evident", how a mathematical concept from Euclid applies to the basic rights of humanity spelled out in the Declaration of Independence, and you can see him making the decision to keep the delegates away from the city in order to delay their arrival until the Amendment can pass.
For the nation: the Thirteenth Amendment passes. The argument over slavery is answered. For all time.
Just to be clear, slavery was (at least arguably) the central reason for everything the government had done up to that point. State borders, the resolution of the Mexican-American War, taxes, voting rights, import duties, expansion of the west, etc, etc, and its over for good.