I've contributed a LOT of answers on Stack Overflow, but I think some of the content I'm most proud of are some of the edits, where I took a complete trainwreck of a question and managed to put myself in the asker's shoes, figure out what they needed, and rewrite the question in a way that actually made sense. Sometimes I even went on to write a valid answer, but not always.
Next in the line would be SQL performance questions. I have a few answers where the OP came back and said via comment my answer took a query that used to run for hours or minutes and reduced it to seconds or milliseconds. The answer wasn't always even that great, in terms of composition, structure, or some finer points of code quality... but when it came down it, it got the job done in a big way.