To me a great answer is one that is 100% correct, but also has some degree of deeper knowledge than maybe anyone else had thought of, or has some added info that none of us even considered.
Or it might be an answer that is correct, but so simple and to the point that none of us longer-winded folks (like myself) could have could have ever said so succinctly.
Or it might be correct, but it may also have a double meaning in which the asker of the Q may or may not get. Sometimes these types of answers can be very witty/funny or even a bit sarcastic, but they are always 100% correct, with a little or a lot of humor thrown in for good measure.
Or it might be a correct answer that due to the expertise of the answerer, was so perfect and so smart that he/she was the only one qualified to answer that particular Q. And it gets me to thinking, “Hmmm, wish I was that smart.”
Or another example would be a very well thought out compassionate answer that actually helps the person asking the Q, even if the asker’s Q was not well written, and therefore subject to some ribbing or teasing from the collective. Some badly written Q’s are just idiots or trolls looking for attention, but some badly written Q’s are just regular folks that might be having a hard time wording a question or forming his/her thoughts because they are in pain or don’t speak English well or have some other problem that makes writing difficult. The best answers are able to figure out what is really going on.