The reason that the upcoming election is always the most important election ever is that from the point of view of today, the only one we can take, it is the only election where the result is unknown and can change the world differently from the way it has already been changed.

From the point of view of the voter about to vote, the first election (where very few people were allowed to vote) or the 1860 election (where the future of the republic was at stake in a way it has not been since) were quite important, but the outcome not in doubt.  Same with 1932.

There are three in my lifetime (only one in which I could vote) that seem "critical" in retrospect, and might have at the time.

In 1960, the candidate who won a very close election on which the future of our country was at stake, said, in the first debate of the general election:

[W]e have to move again in the sixties, ...the function of the president is to set before the people the unfinished business of our society as Franklin Roosevelt did in the thirties, the agenda for our people - what we must do as a society to meet our needs in this country and protect our security and help the cause of freedom. ... I don't want historians, ten years from now, to say, these were the years when the tide ran out for the United States. I want them to say these were the years when the tide came in; these were the years when the United States started to move again. That's the question before the American people, and only you can decide what you want, what you want this country to be, what you want to do with the future. I think we're ready to move.

It seems to me that that comment defines what is a "crucial election".  That one came out the way I would want, but the president was murdered only about 1000 days into his administration.

The other two that qualify as watersheds in my lifetime are 1968 (just look at the events of that year) and, as it turns out, 1980, when the country went, in my opinion, toward its dark side.

This one qualifies, too, at least from this vantage point.  Have you noticed the demise, permanently I hope, of the claim that there is no difference between the candidates and no election matters?

OK, here's my final map
 
Sen Obama should win Ohio but, just to be conservative, I will put in it in Sen McCain's column, on the basis of race, and electoral fooling around. Missouri, Georgia and even Arizona could also go to Sen Obama, but for prediction sake, let's leave them as they will more likely fall