If I could only keep one light, it’d be my ZL H51. It’s by far the most useful light in my collection, and all the others are just toys.
Sure, the SC52 is brighter and dimmer and more efficient, but I have to hold it somehow in order to use it. With the H51 I can just clip it somewhere or wear it as a necklace or headlamp or on the back of my hand or on the car’s sun visor or on my bike handlebars or stick it to a metal surface via magnet or … whatever. I almost never need to actually devote a hand to holding and aiming the light. With its spot+spill and wide output range, it’s useful both up close and at a distance (unlike flood-only headlamps), and it generally just puts light where I need it with no fuss.
For maximum utility, I recommend a couple trivial mods. One, get a small neodymium magnet for sticking it to metal surfaces via the clip. This provides full 180+360 degree aiming. Two, get a shoelace, a piece of double-sided velcro, and a cable lock (holds drawstring bags closed). This can provide a more versatile replacement for its included headband, so it can be worn as a necklace or attach to pretty much anything less than a foot in diameter. It’s also smaller/lighter than the normal headband so it’s easier to EDC and doesn’t interfere with the clip.
I should probably post a thread about Zebralight headlamp mods/tricks.
Edit: I posted that thread: Zebralight headlamp tips / tricks / mods