I don’t know of the most romantic thing, but I know there are a lot of romantic things at WDW.
The “romantic” side of WDW typically doesn’t happen in the parks. The resorts are great places to go for romantic evenings. You could sit on the beach at the Polynesian to watch the fireworks, rent a boat from the Grand Floridian for a fireworks dinner cruise for two, or have a spa day at the GF or Saratoga Springs. The most romantic restaurants are also at Disney’s deluxe resorts. California Grill, atop Disney’s Contemporary Resort provides a great, romantic view of the World, including MK’s evening fireworks. Each night, they turn down the lights and pipe in the music, providing one of the World’s best and most romantic fireworks viewing locations. Narcoosee’s at the Grand Floridian is similar (only not on top of the resort), providing a great fireworks viewing location. Artist’s Point at the Wilderness Lodge is also lovely, as well as Jiko at Animal Kingdom Lodge. And then you could always take an evening stroll around Bay Lake (well… not all the way around, but b/w MK and the Contemporary, and if you walk from MK in the direction of GF, you can find a nice, quiet spot) and watch the electrical water pageant.
The most romantic resorts, in my opinion, are
Grand Floridian Resort and Spa
Animal Kingdom Lodge
Saratoga Springs Resort and Spa
Good luck having a romantic time at WDW!