Depending on the situation this is often number 1.
The first piece of equipment I’d want even before clothing in some cases.
You might be stunned at this preference, I was when I first heard this, but it makes sense.
There are many environments (arctic, desert, beach, ocean, lake , river) where glare can be severe and you will basically be blinded by the reflected sunlight in a few hours. Once you are blinded you have lost your primary sense in perceiving the world, you have lost your ability to detect threats and opportunities, you are in enormous trouble and now you only chance is being aided by others. (If no others come your chances are slim)
Sunglasses don’t have to be expensive to offer good protection from the sun. Generally there is very little difference in this with respect to price, so there’s no real reason to pay more than $10. At $10 or less its quite conceivable to buy two pairs. Sunglasses are like cats and plastic lighters, you never really ‘own’ them you just posses them for a time, sunglasses usually go missing or get broken, don’t waste your money on expensive sunglasses.
There are great debates over this but I prefer all plastic glasses. plastic frames and plastic (polycarbonate) lenses. Less likely to cut me up if they get broken on my face.If they do break they are easier to fix with tape that metal frames. The plastic in the lens of plastic glasses is usually polycarbonate which is very strong and blocks out almost all UV light.
Metal frames might be able to be bent back into shape if damaged.
Metal frames could also be a source of thin metal for fish hooks.
Glass lenses might be able to start a fire if they are correcting lenses.
Glass lenses may also be a source of sharp broken glass (cutting tool) for better or worse.
If you are wishing to be incognito, tinted or mirrored lenses will help, as most systems to recognize faces rely on seeing eyes, this includes artificial systems.
A mirrored sunglass lens might be able to be used for a heliostat for signaling. The sudden and repeated flash from a mirror can be seen for many kilometers in the right conditions. It has been documented that a heliostat mirror was seen from about 30Km in a rescue situation.
So in short sunglasses protect you eyes and preserve your best source of information about the world around you.