Best places in which to celebrate Halloween:

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.
   

WeHo Carnaval was cancelled for this year per their website. It was cancelled last year as well.

"Best" is in the eyes of the beholder...

I consider Halloween a children's holiday, so "best" in my opionion is a school costume parade, and neighborhood trick or treating where the families all know each other.

Your definition of "best" may be different.

These celebrations are designed mostly for adults, not children I believe.

I would add to this list Boston's Cutler Majestic theatre. It is full of apparitions, like a floating couple, a "nightmare room," a little girl... Visitors leave gifts for the little girl, and even the staff say "excuse me" when they pass an empty seat

Denver ghost tours are also a good activity.

I agree with poster J62!!

Philadelphia's Fort Mifflin looks creepy to me. From Revolutionary War soldiers to Yellow Fever victims and Civil War POWs, it's assembled quite a crowd of restless souls over the years

New Hampshire pumpkin festival - Laconia NH

My daughter’s neighborhood in San Francisco: Many of the houses set up outdoor Halloween displays. Halloween night is so busy that two blocks of their street are closed to traffic. It’s a great time to set up chairs in the driveway and visit with the neighbors making the rounds. Last year they gave out 875 pieces of candy.

From that list, I'd like to go to #1.
#4 will definitely be "wild". Fun, though.