When Gatsby demands that Daisy renounce her love for Tom and say that she never loved him, we are witnessing the culmination of conflicts for both Daisy and Gatsby.
Never is Gatsby closer to achieving his dream than at this moment. For Daisy, this is the moment of decision. She finds that she cannot undo history like Gatsby wishes to do. Her life must have some meaning, both past and future, whereas Gatsby is prepared to completely throw away the past in favor of a sought after future.
His character and hers are both brought to their climactic articulations in this scene, with Gatsby demanding that Daisy choose the future and Daisy unable to let go of the past.
I would suggest that the zenith actually starts BEFORE they head to the city in Chapter 7. There is such an interesting undercurrent at the table when they are all at lunch at Tom and Daisy's home. Daisy is flirting with Gatsby in the most subtle, yet obvious of ways, and Tom is 100% tuned in to the dynamic between them. The day is hot, but Daisy comments to Gatsby: "You look like the man in the advertisement." We don't know exactly what she referring to, but Tom must because he gets visibly upset and abruptly calls for the trip into the city. He wants to disrupt what he sees going on at the other end of the table.
I agree. The point is that Gatsby, for all of his wealth and supposed privilege, is always on the outside looking in. Everything is false in his life, and he really does not have the one thing that he truly wants. It turns out not to be something money can buy.
In addition to the interpretation from the previous poster, a reader may also perceive the "zenith" to the love triangle as the point after the irresponsible driving of Daisy in which she kills Myrtle Wilson. The scenerio of the chivalrous Gatsby standing outside the Buchanans' window as they plot their accusations of Gatsby as the driver/murderer certainly underscores Daisy's and Tom's amorality and treachery that explodes Gatsby from the triangle.
The "triangle" as you term it between Tom, Daisy and Gatsby is based on the marriage between Tom and Daisy, but Daisy's evident lingering affection for Gatsby and his fervent devotion towards her. Of course, let us remember that Tom is no wronged husband in this matter. He has been adulterous throughout his marriage to Daisy, and during the time of the novel is carrying on a long term relationship with his mistress, Myrtle. The "zenith" or climax of this relationship comes in Chapter Seven, when the three and Jordan and Nick go to a hotel in the city in a terrible heat that acts as a kind of symbol of the heightened emotions. This turns into a kind of fight for Daisy's affections, and Gatsby forces her to tell Tom that she doesn't love him, but at the end of the day, Daisy is unable to leave Tom, saying:
"Oh, you want too much!" she cried to Gatsby. "I love you now--isn't that enough? I can't help what's past." She began to sob helplessly. "I did love him once--but I love you too."
It is this chapter that makes it clear that Daisy will never leave Tom, in spite of his affairs and her attraction to Gatsby. Thus this is the scene which represents the climax in the relations between the three characters.