bottom of the 9th... 2 outs... bases loaded...

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.

Only two times has a team come from behind in the last inning in a winner-take-all game of a World Series.

1912: The Red Sox score two runs in the 10th to overcome a 3-2 Giants lead to win Game 8 (there was a tie game earlier in the series), 4-3. The winning run scores on a sacrifice fly.

2001 - The Diamondbacks score two in the 9th against the Yankees in Game 7 to win 3-2. The winning run scored on a single.

The 1997 Marlins were trailing in the 9th inning of Game 7, but scored one run to tie. Then won in the 11th.