There isn't one perfect UI, it depends on the purpose of the light:
THIS. A thousand times, this.
I have written about this before, but a tactical light should have, ideally:
1. One mode—maximum brightness
2. One mechanical on/off activation switch
3. One mechanical lock on/lock off switch
A tactical light with a confusing UI or more than one activation switch or without a lock off/lock on is a disaster waiting to happen. You don't put two triggers on a firearm for a very good reason. High stress, life critical situations are the very times when mistakes are most likely to happen and when the consequences of mistakes are most dire.
A safety lock out prevents accidental activation of a light powerful enough to dazzle and blind, both to prevent accident loss of eyesight and to preserve battery life for when it is most needed, not to mention heat damage, should a light become inadvertently activated inside a container.
A safety lock on prevents accidental deactivation in the middle of an emergent situation, should something cause the light to be dropped.
A utility light should always always always ever only start up in minimum brightness mode, preferably less than 10 lumens, or even 5 lumens, both to preserve battery life and to preserve night vision as much as possible. It should also have a mechanical activation switch and a mechanical lock off mechanism, but does not necessarily need a lock on setting.
It's OK for a utility light to have higher brightness modes, but it must always start up in low power mode first. A utility light is not a light to be used for life critical applications. That is what a true tactical flashlight is for. Conversely, a tactical flashlight should never be used as a utility flashlight, in order to preserve maximum battery life for emergent use.
Professional public safety officers really should have two different types of flashlights, one utility, and one tactical, ideally of differing design, to avoid confusion.
The best tactical light UI on the market, imo, belongs to ASP and some Fenix models, like the PD36 TAC, and hopefully the TK28 TAC.
With the PD36 TAC, the lock off doesn't lock on, and it does try to do double duty as a tactical and a utility flashlight, but the way Fenix did this is actually workable: the rotary switch selects lock out, tactical mode that has only high power (press) and strobe mode (press+hold), while the utility mode turns on in low power mode, and successively higher power mode can be selected from there. I still recommend carrying a smaller flashlight for utility purposes, since the PD36 TAC's lowest power mode is 30 lumens, too bright for many uses.
But, you could couple than with, say, a Fenix E20, for utility use.
