Ghosts exist everywhere. They have been manifest in the darkest cemetaries to the brightest pre-schools. From all of my investigations I have gathered a few sites which (In my opinion) have been the most exciting (and active) places for paranormal activity.
Many houses have histories that can bring about paranormal activity and ghosts. Usually the older the house is, the more activity can be generated. As with any sites older dwellings or building have more of a history which can includes deaths, suicides, strong emotional attachment, and accidents that have caused death.
2. Cemetaries
The name says it all. Cemetaries house the dead both in graves and mausoleums. There has been many reported cases of ghosts and apparitions in cemetaries, graveyards, and old burial sites.
3. Places of Death (Battlefields, places of accidents and death)
These sites are usually where others have had a sudden loss of life and their spirits may still linger at these locations. For more information please read about residual hauntings.
4. Schools, Colleges, Boarding Schools
Usually in schools there are always lots of people both young and old and many accidents. Usually a very good place to start an investigation.
5. Churches
Churches can sometimes be a great place for ghost hunting. Although the owners/staff does not always accept the company of (Ghost Hunters) if ever given the change to conduct an investigation, Do So.
6. Hospitals, Mental Institutions
Hospitals hold alot of memories. They are places where strong emotional attachment abound, a place where dying is all around, and a place for family members to gather. If the hospital is still in operation I doubt that the administration staff would allow Ghost Hunter on the premises but if ever given the opportunity to investigate a hospital I would do so.
All in all ghosts can (and often do) exist everywhere. From the darkest deepest forest to your very living room. This means that ghosts can manifest at traffic accidents, old buildings, houses, parks, basically just about anywhere.
The above list is strictly a guide of places I have found the best activity!
Good Luck