Stand in front of the sheet wall and ignore the thread count signs for a second. Feel the actual fabric between your fingers, then picture it in February at six in the morning when the room is cold and all you want is something that doesn't feel clammy against your legs. That's the real test, not the number on the label.
In an ordinary bedroom, sheets sit under whatever comforter or quilt you already have, so the main thing is color and weight working with it rather than against it. A crisp white or pale sheet set makes almost any comforter look a bit more pulled together, and it's forgiving if your duvet cover and your curtains don't quite match, which they usually don't. Percale, the crisp woven kind, suits warmer rooms and summer best, it breathes and cools and gets crisper with every wash, which is exactly what you want in a south facing bedroom that holds heat. Sateen, the smoother and slightly heavier kind, leans toward cooler months and colder apartments, it has a bit of drape and warmth that feels right when the radiator's working overtime and the room is dim by five in the afternoon. If you're outfitting a guest room or a rental where you're not sure who's sleeping there next, a mid weight cotton percale in white or a soft neutral is the one to start with, it suits most seasons, hides nothing and offends no one, and you can bleach it without worry. Linen sheets belong to a different kind of room, the one where you like a slightly rumpled, lived in look and don't mind ironing being off the table forever, they suit older houses with uneven light more than a sharp, minimal new build.
Buy two sets if you can manage it, one on the bed and one in the wash, because nothing ages a sheet faster than being slept on for two weeks straight while you wait for laundry day. Wash in warm, not hot, skip the fabric softener which coats the fibers and ruins the crispness over time, and tumble dry low or line dry if you have the patience. A good percale set softens over months without pilling, a cheap polyester blend will pill within weeks and start to feel slick in a way that has nothing to do with luxury. Watch the elastic on fitted corners too, that's usually the first thing to give out, long before the fabric itself wears through.
No sheet set survives a toddler with a juice box and a nosebleed on the same night, and at some point you will simply need a spare.