The Legendary skin is definitely the best stuff you can get from the loot box in Overwatch. Recently, redditor danius353 made a survey to estimate the drop rate of legendary skins. He has collected more than 30,000 boxes' results, and the data is keep increasing. If you want to add more data to the survey, the link is here. You can check out the date in table and graph form below(The 95% confidence interval for the drop rate is ±0.28%.)
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