If we could keep one category of gear and throw the rest away, it would be merino base layers. Two things justify the price.
They stay warm when wet. Not dry-warm, but warm enough that a sweaty climb followed by a windy summit stops being the miserable part of the day. Cotton in the same situation is actively dangerous; synthetics manage it, but they manage it while smelling.
They do not smell. This is the one that changes how you pack. A merino top can be worn for three or four days without becoming a problem, which means a week-long trip needs two of them rather than seven of something else. That is most of the weight saving people chase with expensive lightweight kit, achieved by simply carrying fewer things.
What to look for
Weight matters more than brand. Around 150–200 g/m² is the useful range: 150 for summer and as a next-to-skin layer under something else, 200 for genuinely cold days. Anything heavier is a midlayer pretending to be a base layer.
Blends last longer. Pure merino is the nicest to wear and wears through at the shoulders where a pack strap sits. A blend with a little nylon lasts noticeably longer for a small loss in feel — worth it for anything you will carry a rucksack in.
Sizing runs close. Base layers are meant to be snug; a loose one traps air in the wrong way and rubs. Size as you would a t-shirt, not a jumper.
The catch
They are not cheap, and they eventually get holes. We reckon on two to three years of regular use before a top is relegated to gardening. Given how much of that time it is the only thing we wanted to wear, that has been fine.