Probably not a top contender, but I want to recognize Where is the Longest String - an excel function question asked by Gary's Student who has an Excel silver badge here and more than one excel-related gold badges on stackoverflow.
The answer was posted by Scott Craner who has an Excel silver badge and multiple bronzes on stackoverflow.
Gary shows the idea with an image but also includes the methods that were tried - he made it half way to the intended answer, but was having difficultly translating a result to a cell address. I used an answer from stackoverflow's simoco to try to figure it out myself. Given there has been an appearance that su's resident formula experts have been scarce lately, myself (not an expert) included.
Gary's question is clear and concise and not exactly easy to figure out. Scott's answer is beautiful, elegant and expandable. I suspect this question to be one that a lot of future dupes can get pointed to.
There's so much going on in these formulas that several blog posts could be written about it - but that would require a blog post on excel which isn't a huge topic for readers.