Contra one Newton Gingrich, who claims the current election is the most important since 1860, there are several others that stand in front of this one. (Though that's not to say this one is unimportant.)
Obviously, four years later, 1864 was more important. Gen. McClellen might, as Lincoln feared, have undone the Emancipation Proclamation.
After that, none of the elections for the rest of the 19th century were that important. One, 1876, was certainly controversial. But, since Democrat Tilden would have ended Reconstruction had he won, it wasn't that important in its results.
1912 might stand as the next truly important election. Wilson, standing for a quasi-Progressivism filtered through Democratic orthodoxy, beat out TR, standing for a quasi-Progressivism filtered through ... TR. Taft was in the dust. Debs drew decently as the truly progressive Socialist candidate. More importantly than any of this, though, arguably was TR hijacking truer non-socialist progressivism of LaFollette and others.
I won't call 1932 that important, really. FDR was clearly going to win. It was important in what he did while winning, so, it was important in that way, of course.
Next? 1948. Had Dewey won, and had coattails to keep a GOP Congress in place, more of the New Deal would have been undone. Less Fair Deal would have been enacted. Armed forces desegregation might have waited years. Had Henry Wallace been FDR's Veep in 1944, would Dewey had won a three-way race, assuming Thurmond still bolting the Dems?
Next? 1960. But NOT for the Camelot myth. Rather, we got the only president to have an inaugural address 100 percent devoted to foreign policy, one who ramped up the Cold War and many other things.
Next? 1980, of course. But, paired with it, 1976. Carter truly parlayed his "outsider" claim to the hilt in 1976, but, it was real enough that his frayed relations with a Democratic Congress hurt him even before the Iranian hostage crisis.
Actually, in many years, primaries have been more important than general elections.
1864: Lincoln accepting necessity, the alleged necessity of War Democrat Andrew Johnson as his Veep.
1896: Bryan watering down Populism through the filter of Democratic orthodoxy. See Wilson in 1912. On the GOP side, "Czar" Reed refusing to stoop to the level of McKinley/Hanna.
1912: The Taft-TR fight.
1932: The last Democratic primary with the two-thirds rule, which forced FDR to accept Garner as his No. 2.
1944: The Democratic Veep fight.
1952: Tricky Dick Nixon undercutting Calif. Gov. Earl Warren's chances against Ike in exchange for the Veep slot.
1976: The Ford-Reagan battle, and myriad Democrats fighting to take on a wounded Ford.