This may not be fair, since I've traveled a bit in my life, but I've visited English churchyards, Irish Graveyards and an ossuary in Denmark, where the bones of the faithful were all faithfully gathered and turned into the decor of the church nave.
Here in America, the top three are fairly legendary on the scary scale.
The first is the coastline north of Boston, Mass. Salem, Marblehead and Glouster all have Puritanical tombstones. These are the classic flat slate, usually inscribed with "Here Lyes y Body of..." Heavily scrolled or floral sides rise up to the arched top, which often sports a winged skull, or a skull and thighbones. The third most popular image on the top was a winged hourglass, because 'time flies'.
The second was Sleepy Hollow, NY. Like the New England boneyards, this burial ground had a few Puritan-style stones, but these showed the changing tastes of the time. The winged skull with its empty, staring sockets is often replaced with a more natural face topped with hair, a representation of the soul ascending... but these graves clustered around the Old Dutch Church are not the main attraction, even though you may be standing near the unmarked grave of a nameless Hessian.
The real treat is the crypts set into the hill on the far side of the churchyard, where Sleepy Hollow Cemetery proper begins. These charnel houses are old, and each has a distinct character - green-tarnished doors, marble pillars, frowning facades and even turrets and embattlements on the top, giving the impression of a castle entrance. One of these crypts served as the original Crypt for Barnabas Collins from the original "Dark Shadows" series.
The third is the cemeteries of New Orleans. The infamous city has a rep for interring its dead above soil due to the high water table, so crypts, sepulchers and mausoleums are popular. The St. Louis #1 is the most famous, with Marie Laveau constantly entertaining guests. There are two others named for St. Louis - #2 is in a bad part of town and in terrible shape (which admittedly draws lots of photographers) and #3 is the newest and best kept, with marble still shining in the swamp-hazed sun. St. Rochs#1 can be decidedly unnerving, much more than #2, and Lafayette #1 has a wide range styles because it's non-denominational, and popular to boot, as it's in a nicer part of town and was made famous by Anne Rice.
But the creepiest, most skin-shivering grave? It was the grave of L. Coombs in the churchyard of Littleton, Cambridgeshire, England. That was the town I lived in while stationed in England, a small burg just up the road from Ely and Cambridge. Coombs had been buried before the Declaration of Independence was written, and the gravestone leaned out over the wall of the churchyard and cast a long shadow on the sidewalk beyond.
One evening, while walking past it, I noticed something in the grass of the grave in the light of the nearby streetlamp. I stepped up and looked closer, then carefully lifted the object from the earth. I brushed the soil from it and, guessing what it was, walked it up to the church, where the sexton met me and thanked me for bringing my find to him.
Like many churchyards in Europe and the British isles, space was at a premium before the rise of secular cemeteries. This led to many families being buried in the same grave, one on top of the other. Usually, the previous occupant was long gone when the graves were opened for a new resident, but nevertheless some graves still swelled to capacity, so the recently departed may not have been assured a full depth of six feet. He figured that was what had happened at the Coombs grave, because the object I had brought him was the iron ring handle of a coffin, covered in a thick coat of rust.