I may be older than most of you...in order from when I was a kid:
Ebbets Field
Polo Grounds
Old Yankee Stadium
Connie Mack (a real dump)
Shea
Old Memorial Stadium (in Baltimore I actually like it more than Camden Yards)
RFK Stadium (but for DC United)
Fenway
The Vet in Philly
Dodger Stadium (one of the best)
Three Rivers
Kauffman Stadium (probably the best)
Oakland Coliseum
Atlanta Fulton County Stadium
Fenway
Wrigley
Comiskey (the new one now called US Cellular--probably the worst)
Camden Yards
Citizen Bank (I think thats the name of newest Philly stadium)
Nationals Stadium
Minor League games in Frederick MD, Chattanooga, Greensboro NC, Prince William VA, Hagerstown and a few others I forget now.
A few spring training games in FLA (Port St. Lucie, West Palm and Vero Beach--the best place to watch a game.)