The one I reach for is the one already on the chair from yesterday, not folded, just there, when I've got twenty minutes and need to look like I made an effort without actually making one. Under a jacket for the school run, on its own when the heating's finally on, it's the thing that gets worn before I've properly woken up and decided what I think about anything.
Where it goes wrong is pairing a thin, clingy one with anything else fitted, so a soft jersey tee under a structured blazer can look like you've forgotten a layer, and a cropped one with high waisted trousers only works if there's actually no gap, otherwise you're flashing a stripe of midriff every time you reach for something on a high shelf. Graphic ones clash with anything else trying to say something, a loud print and a statement necklace is one too many ideas for one outfit. Keep the rest of you quiet and let the shirt do its one job.
Fit is where most of mine have gone wrong over the years. True to size usually means true to size on the body, not on the label, so check the shoulder seam sits on your shoulder and not halfway down your arm, because that's the thing that makes an otherwise fine t-shirt look like you borrowed it. Cropped ones need trying on sitting down as well as standing, since a length that's fine standing can ride up to nothing the second you fold into a chair. Go a size up if you want it to skim rather than cling, a size down if you're after the cropped, slightly boxy look, but don't expect one t-shirt to do both.
How it ages depends almost entirely on the fabric weight you started with. A thin, cheap cotton goes see through at the shoulders within a few washes and the neckline stretches into a sad little scoop. A proper heavier cotton, the sort with a bit of body to it, keeps its shape for a good year of regular wear, washes better on a cool cycle, and softens rather than thins. Turn dark ones inside out before washing, skip the tumble dryer if you can, and check in the shop whether the hem is a proper double stitch or just overlocked, because that seam is the first thing to go.
It's the thing that makes a pair of jeans you've had for years look like you dressed on purpose this morning.