Here's my ranking of the current 17 ballparks that I've visited:

1.) St. Louis- New Busch Stadium
2.) San Francisco- AT&T Park
3.) Chicago- Wrigley Field
4.) Boston- Fenway Park
5.) New York- Citi Field
6.) Atlanta- Turner Field
7.) New York- New Yankee Stadium
8.) Pittsburgh- PNC Park
9.) Kansas City- Kauffman Stadium
10.) Anaheim- Angel Stadium
11.) San Diego- Petco Park
12.) Cleveland- Jacobs Field
13.) Arizona- Chase Field
14.) Los Angeles- Dodger Stadium
15.) Chicago- U.S. Cellular Field
16.) Toronto- Roger's Centre
17.) Oakland- Oakland Coliseum

Baseball Stadiums That I've Visited (With Most Recent Year Visited)
Anaheim (2009)
Arizona (2008)
Atlanta (2009)
Boston- (2007)
Chicago- Northside (2007)
Chicago- Southside (2007)
Cleveland (2007)
Kansas City (2006)
Lake Elsinore- A Minor League (2006)
Long Beach- Independent Minor League (2008)
Los Angeles (2009)
Minnesota (2004)
New York- Citi Field (2009)
New York- New Yankee Stadium (2009)
New York- Shea (2008)
New York- The House That Ruth Built (2008)
Oakland (1990)
Peoria- Spring Training (2008)
Phoenix- AAA Minor League (1989)
Phoenix- Spring Training (2004)
Pittsburgh (2007)
Sacramento- AAA Minor League (2006)
San Diego- Jack Murphy (2004)
San Diego- Petco Park (2009)
San Francisco- AT&T Park (2008)
San Francisco- Candlestick (1990)
Scottsdale- Spring Training (2008)
Springfield- AA Minor League (2006)
St. Louis (2006)
Tempe- Spring Training (2008)
Toronto (2009)
Tucson- Spring Training (2008)