NL - Shea Stadium
AL - Old Yankee Stadium, Memorial Stadium (Orioles), Camden Yards and old Tiger Stadium. Have been told Nats Park is very nice though I haven't been there yet.
I just noticed that of the 5 stadiums I listed, four no longer exist.
Also been to several minor league stadiums as well - BB&T ball park (Winston-Salem), Stadium where the MD Blue Crabs play, Bowie Stadium and Potomac Nationals stadium in Prince William County.