Eveready AA LED penlight in left front pocket, outside edge. Use it all the time. Looking at code numbers on locks, looking into locks for what went wrong this time, Just looking in general. Great for finding something which went on the floor and is itsy bitsy and now you find that stupid part which had to escape.
Never walk into a dark room without first shining the light around. One never knows what lurks in there. It could be a drop off of a foot or so - open buss bars which look like supporting steel - high pressure anything. Once I opened a long closed and locked door only to find it was 15 feet off the floor below. Nobody remembered where that door went to and there were no lights in that room. I was sent out to find out. It turned out to be an old coal bunker which has long been closed off and forgotten, except for the door which now was going get a stairway and the room used for a storeroom. (I carry all the old system keys on my work rings, so if a door like this pops up, in the end I have the ONLY key for it.)
In my SAK SOS kit, I have a Maglight as a back up. One can never have too many lights with them on the job, and although the LED light is brighter, any light is better than no light.