Lilly's blindside of Marc.Just before Tribal Council, Lilly finds a "hidden immunity idol" under the tribe flag, which is actually fake and planted by Minerva's alliance, and plans to use it that night. When the time comes, she pulls out the idol and uses it (while Chaona face-palms, since she knows it's fake), but of course, it's fake and it looks like she's doomed... that is, until she suddenly reveals the real idol she found on exile and plays it, and blindsides the other alliance by voting out Marc. It changed the whole course of the game when Barney managed to flip Luke on his side afterwards.
Lilly's bait and switch with the HII in Fans vs. Canons. She gives it to Barney at the last minute after promising it to Minerva. Minerva's face is pretty scary after that, but hilarious.
Ker:When looking at you, I thought I saw what you needed to become to win the game, and I didn't want that. Thankfully you never won the game. So there is still hope for me to win this game without becoming anything close to you.
Sunflower, when forced to choose between voting out a good friend or breaking her promise to her alliance, Takes A Third Option. She gave her word that she'd vote for Eli, but not that she wouldn't play her Hidden Immunity Idol on him. As a result, Flaithri is blindsided and the Hantz underdogs are in a powerful position when they seemed certainly doomed only a moment before.
Kim and the other heroes turn the tables on Enter. Kim figures out that one of his idols is invalid and uses a Batman Gambit to make him play it on his closest ally, Bowser. When Enter realizes what happened, he literally bluescreens.
It was a Xanatos Gambit, actually, on closer viewing. Once Enter played both his idols, either Bowser (if he didn't take the bait) or Yvonne (if he did) was going home. Either way, Enter's team continues to dwindle.
Iyzebel's comeback in Season 13. After having been on the verge of Rage Quitting until Alleio gave her some words of encouragement, she takes the reins for the next challenge and completely dominates it.
Alleio, after admitting to sabotaging his tribe, doesn't use his idol, only get ONE vote and the very "traitor" he wanted gone, Annabelle is voted out in a 3-2-1-1-1 split.
Bonus point: The only vote he got was from Annabelle, none of the remaining players still in the game voted for him and he still has his idol.