Solitaire: POS; puny amount of light, and the switch is unreliable unless used frequently. Also tends to turn on when you don't realize it, and battery is thus dead when you need it.
2 AAA, 2 AA: obsolete; poor amount of light for weight and bulk. I had hoped to be able to improve on these by having Carley make up some special order xenon star lamps for these sizes, but have been able to do so. They keep making them, and they don't fit.
2D: OK if space and weight is a concern, but you should probably be using a Sure Fire if this is the case.
3D: best all around size; good amount of light with the Carley xenon star lamp. Excellent balance.
4D: starting to get bulky and heavy; best size if you are using KSR (rechargeable) lamp with NiCds. There is more light than the 3D, but rarely does this allow you to do something that you couldn't do with the 3D. From here on, you get correspondingly less light for the increase in size and weight.
5D, 6D: not useful in my experience; too heavy and bulky; you tend to not have it with you. If you are using one of these, I suspect you have not discovered xenon lamps. A friend of mine is a helo medivac pilot, and he had a 6D (using standard PR lamp). I fixed him up with a 4D with xenon star lamp. Here is what he had to say:
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Greetings, Walter.
I recently landed at a night highway scene and, while waiting for the
nurse and respiratory therapist to bring the patient to the helicopter,
I used the flashlight to recon my takeoff path. I found several poles
the landing zone crew had not noticed and from them inferred the wires
in the dark.
Thanks.
Ken
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The 6D is now a back up.
Hope this helps, Walt