The icing on the cake, or the cherry on the cake is a useful idiom if you are talking about something which is good but you want to say that within that good thing there was something even better. For example, Olympic athletes all want to win an Olympic medal, but the icing on the cake would be to win a gold medal.
Or, on a personal note, playing pool is fantastic, but the icing on the cake is when I win without cheating. This does not happen very often.
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