Having just gotten around to watching Lincoln (Spielberg's great movie), I'm now convinced that the most important amendment is the 13th, which finally abolished slavery.
Even with the Southern states gone, the amendment just barely squeeked through the House of Representatives, and required a heck of a lot of politicking.
If the Southern states hadn't attempted to secede, then there is no way that slavery would have ever been abolished, and the basis of our nation's economy would be human trafficking.