Every year I provide this list I compiled.

1. Salem, MA. This is where the Salem Witch Trials were held 1692-3 and 20 “witches” were executed. The town proclaims its Halloween celebration as America’s biggest.
   
2. Sleepy Hollow, NY. The "Legend of the Sleepy Hollow" written by Washington Irving in 1820 features the headless horseman. The town holds haunted hayrides, Gothic mansions, blazing pumpkins, parades, and the Headless Horseman himself.
   
3. New Orleans, LA. Famous for voodoo and zombies, New Orleans knows how to put on a spooky party. One of the biggest celebrations is on Frenchman Street where everyone is in costume, some of which are flamboyant, elaborate or outlandish.
   
4. West Hollywood, CA. The West Hollywood Halloween Carnaval is a wild costume party scene. Unique costumes by some of the most creative people on the planet.
   
5. Key West, FL. Fantasy Fest is an annual ten-day carnival in a tropical paradise that celebrates creativity, imagination, freedom and music.
   

I also found this:

"New York's annual Greenwich Village Halloween Parade is the world's largest Halloween parade, and it doubles as a pageant where participants can show off their wild and creative costumes. The real stars of the parade, however, are the larger-than-life puppets operated by volunteers."

We were in Salem MA for a friday afternoon food tour in late September. The guide wanted to be done by 4pm because it was Mayor,s Night. Parents brought young children to Trick or Treat in the downtown area. We had an early dinner on an outside patio where it was fun to watch the costumed families. A lot of parents also dressed up. During the day, even late September, we saw people dressed as witches and warlocks. If you want to enjoy Halloween in Salem, this would be a much quieter time to go. Free parking before October 1 was also easier to find.