Ok, I am going to take a swing at this one even though I am not in my 50's, 60's or 70's. I am only in the mid 40's, but I will risk an answer here.
The most important election in my lifetime is the current one. Of course, in 2000, the most important one in my lifetime was that one. My point is, each election should be viewed as the most important, because it is only during the current one where we can let our voices be heard in order to effect change (or keep things the same as the case may be).
The idea that any particular election is less important than others leads us farther down the path of voter apathy which I don't think is a good prospect.
So I honestly cannot pick one during my lifetime, because at the time of each one I considered the issues of that election important.
See, I just continue to be hard to deal with don't I.