Sydelle Pulaski figuring out that the clues are the lyrics to "America The Beautiful."
And having the forethought to write her copy of the will in Polish in case anyone stole it.
She didn't just write it in Polish. She wrote it in Polish in shorthand. She essentially forced anyone who tried to steal it to have to crack two barriers to read it.
She even came pretty close to figuring out the whole thing, in that Angela's "to win = twin" theory uses exactly the same out-of-the-box thinking as "The one who wins the windfall is the one who finds the / FOURTH". If the will hadn't been stolen and she'd had more time to review the original text, Sydelle might have solved the whole thing!
When the clues are passed out, instead of immediately opening hers, Sydelle has Angela listen with her to how the other teams are reacting to their own clues. That gets them a bit of an advantage later, since they're able to use the reactions to make very good guesses as to what some of the other clues were.
Judge Ford and Sandy McSouthers spending nearly the entire game systematically investigating the backgrounds of the other heirs. And we find out that end that McSouthers - who was pretty much The Watson for Ford - was Westing himself. And he was pretty impressed by Ford!