I've been carrying a Fenix light of one sort or another for some years now.
The first was an L1T V2.0 which was run by a single AA battery. Tail cap button operated with click on and also temporary on by pushing the button lightly. It had two light levels, one about 15lumens for close work and one about 100lumens. The light level was selected by twisting the head so the selection persisted and was solid--didn't change accidentally. I really liked that light, it was compact, very simple to operate, and I used rechargeable AA's so once I was past the initial investment it was very cheap to run.
But eventually the Lithium Ion battery powered models just outclassed it in terms of brightness.
I upgraded to a PD25 which will run off a single CR123A, a rechargeable 16340, or better yet, off of Fenix's special version of the 16340, the ARB-L16-700U that has a micro-USB connector built right into the battery so you can charge it directly with a typical phone charger. It has typical tactical tailcap button operation for on-off and temporary on.
The only downside to the PD25 (and indeed most of Fenix's offerings) is that Fenix apparently feels that if a light doesn't have at least 5 modes, the user will feel cheated. Fortunately with the PD25, the modes are controlled by a separate button on the side of the head. I have not had trouble with the button being operated accidentally and operation is simple. Pressing it cycles through the brightness levels from about 5 lumens (which is really almost too low to be useful) up to 550 lumens in "turbo" mode when using a 16340. Using a CR123 drops the turbo mode to 400 lumens. The light will remember what mode it was in the last time it was used and come on in the same mode automatically.
Runtime (in terms of battery capacity) is very good, even at the high levels, but when you're running the light in turbo mode it will overheat long before the battery runs out. The overheat condition will be detected and cause the light to drop down to the medium light level of 150 lumens. The same is true if the battery gets low--the light automatically drops from turbo to medium. I generally leave mine in turbo mode when I'm carrying it so I have maximum output immediately if I need it and then I can drop to a lower level if I need to.
There is a strobe and at least one other mode, but those are harder to access--you're not going to get into strobe mode accidentally which is just fine with me. But the mode is there for people who want it.