Let's limit this to, say, the last 100 years or at least the post-Civil War elections. Otherwise, I'd pick a bunch of the earliest elections as "the most important".
Heck, even the 1844 election: It was the first election where an "anti-slavery" candidate got a significant number of votes -- about 5% in each of the Northern states he ran in. If not for him, then the Whigs would have won New York and Michigan (and only needed NY) to win the whole election. No Polk, no Mexican War, no union with Texas. Texas would probably have ended up with about half of the white space in the map below.
The annexation of Texas happened during Tyler's lame duck period, not during Polk? And Tyler was pretty damned determined to annex Texas and (for obvious reasons) didn't much care about politicians in either party.
The Republic of Texas was well on its way to failed state status in 1845 anyway. A continuing independent ROT would be losing, not gaining, territory (Mexico might well have been able to force an effective border of the Nueces River and put the Texas/Mexico border just south of San Antonio).
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