I have pretty similar tastes to you in a headlamp, at least from what you have told us. I have still not found what I would consider as an ideal light, but I do have a clear frontrunner. The single AA lights I have owned are an Icon Irix 2, a Mammut (model name escapes me), a rayovac sportsman, and the fenix hl21.
The Irix 2 was decent. It had the infinite brightness adjustment, which IMO leaves every other user interface in the dust, while also being more function in regards to output vs runtime control. It had a cheezy mounting system though, nowhere near enough throw (which is VERY important to me), and it produced some glare in my eyes from the edges of the leds.
The Mammut is a slick little light. It is the smallest and most comfortable of all my headlamps. It has a decent combination of brightness and runtime, with 3 levels of light output, but does not have enough throw for my needs. It also isn't super efficient when on the high mode. I wish one of the 5mm leds was a thrower to give it some range. In the low (single 5mm led) mode, this light is awesome for making enough useable light for average camp and home use while having a very nice run time.
The Rayovac sportsman is old technology, but was my first single aa headlamp. It has good throw and a decent diffuser, but awful runtime, and a bunch of weird colored leds that I have no use for but had to cycle through all the time to use the lamp.
The Fenix HL21 absolutely destroys all the other single aa lamps I have used so far. The low of 4 lumens is enough to night hike on good trails (which is what I use a headlamp for the most) and spot blazes on trees. It is way more light than I want in my tent or to look through my backpack, but at 50+ hours of runtime on low it is a real battery miser. The medium setting is way too bright to use for much of anything unless you love replacing batteries. It could be the high setting, and I would be plenty happy. I wish it had settings of 1 lumen, 10 lumens, and 50 lumens. The high setting is noticeably brighter than the medium setting, but not enough so to be any more useful than the medium setting (even though it is more than double the lumens), because it kills the battery fast, and I don't find 100 lumens any more useful than 45 or so for my uses. It has 3 brightness levels, can have the cap loosened to lock it off in your pack, has awesome throw with enough spill for my uses of night hiking, and will run a long time on low. It is also very comfortable.
If the Fenix just had the brightness knob of the Irix 2, I would not even consider any other lights. My only knock on the fenix is that the light levels are stupidly chosen. They look good on paper, but perform oddly in the field. I can't be the only one who thinks infinity adjustable brightness is by far the way to go on almost any light, but especially a lightweight headlamp for hiking purposes.
