A plain cotton top suits almost anyone, which is exactly why so many of them are disappointing. If you want something that moves with you, washes easily and doesn't need thinking about under a jumper or on its own, this is the right category. If you're after structure, a bit of shine, something that holds its shape without any help from you, look elsewhere, because that's not what cotton does and no amount of clever cutting will make it pretend otherwise.
How it wears over time tells you more than how it looks on day one. A good cotton top softens with washing rather than thinning out, and the neckline stays where it started instead of slowly widening into something that shows your bra strap by October. Cheap ones go the other way: the first wash shrinks them slightly, the fifth wash loosens them permanently, and by the tenth the hem's gone wavy and the whole thing hangs differently on one side than the other. Wash it cool, inside out if there's any print, and skip the tumble dryer if you can, because heat is what kills the shape fastest, not the washing itself. A top that's been through a full season of being balled up in a bag after the gym and still looks the same as it did in the shop is one worth holding onto.
The difference between a good one and a bad one is mostly in things you can't always see from a photo. Heavier cotton, the sort with a bit of weight to it, drapes better and doesn't go see through the second you're anywhere warm. Check the side seams, they should sit flat rather than twisting round towards the front, which happens when the fabric's been cut off grain to save money. Ribbed collars and cuffs should spring back when you stretch them gently, not stay stretched, that's your sign the trim has proper elastane in it rather than just looking ribbed. Stitching on the hem should be even and close, not wide loops that'll catch on a zip the first time it goes in the wash with something else.
In the shop, hold it up to the light to check it isn't thinner than it looks on the hanger, and give the shoulder seams a tug, because loose or wonky stitching there is usually a sign the rest of the construction is just as careless. Try it with your arms raised, the way you'd actually reach for something, and see whether the hem stays down or rides up to your ribs. The good ones don't move. And the real pleasure, once you've found one, is pulling it out of a drawer after a long day and it still smells faintly of the wash, not of the shop.