Which is the most serious invalidator of a fast and requires an expiation?

Intercourse is the most serious invalidator, as it has a severe expiation along with it. The person needs to make tawba, make up the day, and fast for 60 consecutive days. The evidence is in the following hadith:

Narrated by Abu Hurayrah (ra) who said: A man came to the Prophet (pbuh) and said, “I am doomed, O Messenger of Allah!” He said, “Why are you doomed?” He said, “I had intercourse with my wife (during the day) in Ramadan.” He said, “Can you free a slave?” He said, “No.” He said, “Can you fast for two consecutive months?” He said, “No.” He said, “Can you feed sixty poor persons?” He said, “No.”… Narrated by [Bukhari and Muslim]