Not sure if there was a specific beer, honestly. I spent 6 months in Cambridge, England in 1989-90. We frequented a number of pubs during my time there, in particular a small, unassuming place called The Grapes... Six months of good English brew and a month traveling the continent... beer was just beer to me at the time. It wasn't until after I'd returned to the midwest that I realized how good I'd had it in Europe, and how crap the normal swill that I'd been drinking in the US really was.
There was a certain amount of enlightenment that occurred at that time, and the general search for beers with flavor and substance began shortly thereafter. Proud to say that I've been drinking "better" beers ever since.
Thinking back, I think the first microbrewed beer that I actually remember drinking was Pete's Wicked Ale. That and beers from Anchor Brewing.