Almost everything about this episode was top-notch. First of all, the mystery was better than most episodes. Even though we knew who the killer was from the start, the motive was unclear and there were lots of good subtle clues dropped along the whole way.
Second, the comedy in the episode was hilarious. One scene that comes to mind is when trick-or-treaters come to Ambrose's door and he says something along the lines of "By giving you this treat, you agree that you cannot committ and tricks against neither me, my house, nor my property." It was hilarious that he takes such a simple thing like Halloween and treats it so seriously.
Third, this was a great episode for character development. We learn more about Adrian and Ambrose's past when Natalie looks at the photo album (such a sad scene), and Natalie's character also grows stronger here. I like that she didn't flat-out reject Ambrose when he asked her out.
And last, the drama was great. I seriously thought that Ambrose would die when he ate the candy bar. And also, though the Monk brothers reconciled in "Mr. Monk and the Three Pies," there is still tension between them as they wait for their father comes home and Adrian says that he's not coming back. Since their father didn't come back at the end, it also leaves the door open for future episodes.
So altogether, this episode had the perfect balance of mystery, comedy, character development, and drama to make it one of the best episodes of Monk.