I would have to say Dodger Stadium.
I regret never making it to Comiskey and Tiger Stadium while they were around.
I have been fortunate to have been to both Old and New Yankee Stadium (combined--literally hundreds of times, 2x last week), Shea Stadium, Citi Field, Fenway Park, Camden Yards, Wrigley Field, the old Veterans Stadium, and U.S. Cellular Field (White Sox).