When Pam got drunk at Dunder Mifflin's annual "Dundies" awards show, she was so relieved at getting the "Whitest Sneakers" award instead of "World's Longest Engagement" like she got every other year that she kissed Jim. And thus began the shipping of Jim and Pam!
The cold open alone is the reason this episode made the Best list. Jim pretending to be Dwight was done to perfection, and Dwight freaked out just as Jim thought he would. Bears, beets, Battlestar Galactica.
We already knew that Michael was a surprisingly talented manager, but in "The Duel" we found out that the Scranton branch of Dunder Mifflin is actually the most successful branch of the company. Michael Scott, you earned that chance to act smug in front of CEO David Wallace!
Jim and Pam knew their co-workers would disobey them and recreate the "JK Wedding Entrance Dance," among other disastrous events during the ceremony, so they took off and got married on a ferry boat underneath Niagara Falls before the ceremony. It was beautiful and perfect for Jim and Pam.