Vital for moving around in the dark! There are many types, however, my bug-bear is 'normal' torches! Please!!! If I get you to change one thing during an invest it would be this; please please invest in a red lens or red LED or filter! for your torch, It means you don't get blinded by white light in your face by the person who always forgets to keep their torch pointing at the ground. There is also the very practical point in that it take your eyes roughly 20 mins to adjust to the dark, everytime someone turns on a bright torch, there goes your time spent adjusting!
My personal favourites are a head torch, however in the interests of not looking a complete dork (technical term!) instead of putting it on my head, I wear it on my upper arm, you can then still use the beam to see where your walking and it's easy to find! My second favourite is my trusty maglite with red filter. (other brands of torches are available).
As as a side note, there is no reason not to do investigations during daylight. In fact more 'ghost sightings' are reported during people's normal daily activities but it's not as much fun in the daylight and thats why Ghost Hunts are carried out at night as its more atmospheric and there is less noise polution.