I am assuming this is restricted to stadium concessions (excludes tailgating), the best I've had is the BBQ Pork at Minute Maid, and a sausage and peppers sub at Coors.
The burgers at the then Prince William Cannons games, circa 1990, are still my favorite ballpark burgers, and still beat out all spring training, minor league, and pro stadiums.
I want to try Boog Powell's place in Camden some day. I've heard great things about it. Also, I've heard great things about Kauffman Stadium in KC.
Pro stadiums I've been to: Dolphin Stadium, AT&T, Busch II, Mile High, Coors, Dodger, Miller, Minute Maid, RFK, Jack Murphy, Memorial, Yankee, Riverfront, Three Rivers, Cleveland Muni, Jacobs, and the Vet.