"The Lost Caverns of Tsojcanth" was the first gaming experience that I had with D&D back in 1991. Many years later ran it for a group as the DM, the campaign log can be found on my P&PG blog. Met two good friends at that group that I still game with to this day.
Nest favorite would be the "Underdark Adventure". Was the first and only time that I have got to start a character from the very begining of its adventuring life. That campaign log is also on my P&PG blog.
"You go 50 feet and come to a Y."