In order by the number of games I've seen at each venue:
Nationals Park
RFK Stadium
Atlanta-Fulton County Stadium
Camden Yards
Candlestick Park
Wrigley Field
Turner Field
Yankee Stadium (previous)
PNC Park
Jacobs Field
Citizens Bank Park
Fenway Park
Busch Stadium (current)
Comiskey Park (current)
Astrodome
Busch Stadium (previous)
Safeco Field
Ballpark at Arlington

Oddly enough, I've also been to the following to see football games, but never for a baseball game:
Shea Stadium
Memorial Stadium (Baltimore)
Joe Robbie Stadium

Mark Hayne
gixc@verizon.net