The answer to this would depend on whether you are asking about the things that the saint did that can be accepted by secular historians or if you are asking about things like miracles that are accepted by the Church.
If you look at this from the point of view of the Church, St. Anthony has so many miracles to his credit that it is difficult to pick the most impressive one. One that might be picked as the most impressive would be the one in which he restored the young man's amputated foot.
Looking at St. Anthony simply from a secular standpoint, his most impressive events have to do with him standing up to the powerful and supporting the weak. For example, there are stories of him standing up to and reproaching high church officials and of him pushing the government of Padua to pass laws protecting debtors.