My vote goes to Mr. Koch and Samuel Adams Boston Lager... I was pretty much an MGD/Corona guy before then. ...
Similar experience... was a Budweiser guy in the college days. Shortly after someone tossed me a Sam Adams Boston Lager and the rest was, as they say, history.
Although some beers have been more influential since then. For example, Cantillon's gueuze really opened up my eyes to how a lambic should be.