Of all the features a flashlight might have (Lithium vs. NiMH, Warm vs. Cool tint, Clicky vs. Twisty), the one critical feature that I find most defines a flashlight from my perspective is the ability to tailstand well. Note I said well. A friend once explained that he had coached his wife to run her first marathon and she had run it well. I knew what he meant. He didn't mean she won the speed record. He meant she had energy to spare at the end, was able to enjoy a celebratory dinner out and make it to work the next day without limping. This is what I ask of a flashlight. A little committment to tailstand as a civilizing feature.
Before I continue I should note that many have indicated this to be of little or even no importance to them in their flashlight selection. I have heard that folks never tailstand their lights, they lean them if necessary or put them in a water glass if obliged to have them face upward. For all of you, I respect your positions but your experience differs from my own. I rarely use a flashlight without, at some point in the evening, needing to tailstand if only briefly.
So. let's talk about configuration. Flashlights without flat ends just don't tailstand. Ok, we all know the HDS Clicky / Rotary with flush button will tailstand under duress. It's heart really isn't in it though. The Jetbeam PA40 is another one. If you put the lanyard bean under the short side it will stand reasonably but it would rather not IMHO.
For the lights with flat ends, some are just too wobbly or with too high a center of gravity to count. Some have crenelated tailcap shrouds which inhibit stability. Examples of too wobbly are almost all 2AA configuration lights and surprisingly most 1AA lights. 18650's do better but some are throwers with really heavy heads (e.g. Armytek) and just aren't very stable. 123A lights are more likely to succeed but even some of them disappoint. Even nice wide lights will sometimes fail because they are really tall. I'm not sure I would feel comfortable with a tailstanding TK50 but your mileage may vary.
So what are the flashlight tailstanding champs? Here's the list from my collection:
Preon P0 due to it's magnetic base on metal surfaces only
Quark Mini 123
Peak Logan 123 with flat bottom
Malkoff MD2 version 1 with delrin shroud (nice and wide despite tall body)
Multi Cell Category
Fenix TK45 (just about reaching the top of the champ height for the diameter)
Anyone want to add to the list?