Pretty much all of Emily's takedowns of her targets:
Emily's comprehensive takedown of Lydia, not only destroying her friendships and getting her banished from the neighborhood, but making sure she gets nothing in her divorce, when she and her husband were equally unfaithful. Plus, she set the husband up with the woman he cheated with.
In the following episode, how she plays Bill Harmon and utterly destroys him and his company.
To elaborate, she leaks to the press that Nolan will be giving an exclusive contract to a specific cellular company. Harmon hears this and tells his employees to invest everything in said company. Turns out, Nolan never intended to give the contract to that company. In fact, he was planning on handing it to their rivals. Harmon loses several billion dollars worth of investments and to put the icing the cake, Emily ruins his attempt to cover it up by personally calling every single one of his investors to inform them of the development.
Ruining the political career of a senator who years ago prosecuted her father and refused to admit evidence that would prove his innocence in order to win the trial.
Paying back one psychological torment with another, when she kidnaps the psychiatrist who pressured her into believing the lies about her father and then anonymously calls the police, leaving the doctor to spend the rest of her life wondering who it was and if they'll ever come back.
Plus, destroying her business by stealing the tapes of her sessions and putting them online, including her own sessions so no one will suspect her.
It actually gets better, given one of the doctor's patients was Victoria, and that her own "confessions" she made to the doctor were all about how she was falling in love with Daniel but afraid to get close to him. So by revealing the tapes, she (a) Destroys the doctor's practice and insures she'll never get clients from the Hamptons set again, (b) Humiliates Victoria and drives a wedge between her and Charlotte, (c) Furthers her seduction of Daniel by letting him see her "embarrassing confessions." Not bad for an afternoon tea.
Burning down Mason's house, starting with his treasured only copy of his memoirs, while leaving behind "Amanda's" lighter to implicate her and get rid of a Spanner in the Works. And she gets away with her father's interview tapes.
Victoria smiling to Lydia and hugging her as she delivers a brutal verbal takedown:
Lydia: "I am so happy that we're friends again."
Victoria: "Well it certainly appears that way, doesn't it? Then again, appearances can be deceiving, can't they. You've practically made it an art form. Understand something, Lydia: every time I smile at you across a room, or we run into each other at a luncheon, or I welcome you into my home, let that smile be a reminder of just how much I despise you. And that every time I hug you, that warmth you feel is my hatred burning through."
The real Emily (who Amanda switched identities with) takes Frank down with a tire iron, just as he's about to reveal Amanda to Victoria.
When Grandpa Grayson verbally eviscerates Conrad on his failure as a father.
When Tyler is being arrested for attacking Daniel's birthday party, it's revealed that Emily took advantage of the situation to frame him for Frank's murder as well. Now, bear in mind that when Tyler showed up, Emily thought he had left town for California, and yet she still managed to put together a plan to put him away for a long time on the fly. I think it's safe to say at this point that Emily is a master of Xanatos Speed Chess.
Mason Treadwell's Heel-Face Turn, destroying the Graysons' defense plan for Daniel and then telling Victoria he's perfectly willing to reveal their framing of David Clarke and go down with them. "It's a small price to pay for restoring my soul."
Emily's brutal beatdown of the thug Victoria hired who beat up Jack and had Daniel assaulted in prison.
Emily improvising to frame Lee for Tyler's murder — in doing so, she gets Daniel out of jail without implicating Jack.
Emily's fight with the White Haired Man. The guy is a trained assassin with what must be several decades of experience on her, and she still kicks his ass. And then she lets him live, because that's what her father would have wanted. True, that last part blows up in her face, but it's still impressive as both a fight and characterization.
Emily finally goes one better in season 2, episode 6, wherein she frames Conrad for Gordon Murphy's death, setting the stage for Daniel to take over Grayson Global, and apparently as part of a plan to deal with Mason Treadwell once and for all.
Any and all interactions between Emily and Mason in "Penance." For both of them.