How about this: remove the restriction that it has to be a grand slam. Let’s say the most dramatic possible moment in a ball game would be that it’s post-season, double-elimination game (game 7). Home team is down by x runs, with x men on base, with 2 outs in the bottom of the 9th. Home run to win the game and the series, and snatch victory from the jaws of defeat.
This scenario hasn’t happened yet.
Close would be Bill Mazeroski, Game 7, 1960–but the game was tied at that point. Neither team was staring at defeat.
Probably a better example would be Bobby Thompson, 1951, “shot heard 'round the world” “Giants win the pennant!”. This fits everything, except that it was a playoff, not the World Series. But it’s why this should’ve been the most memorable moment in baseball when they did that stupid poll last summer.