The first white top I owned was a thin cotton thing from a market stall, and under the shop lights at work it taught me that white has no secrets, you can see everything through it, including the bra you weren't planning on advertising.
If you've none in the drawer and you're starting from scratch, go for a plain crew neck in a proper weight of cotton before you go near anything cropped or sheer or covered in broderie. It sounds boring, I know, but a good plain white top is the one piece that has to work on its own merits, there's no print to hide behind and no colour to distract the eye, so get the simple version right first and the interesting ones make more sense later. I spent years assuming white was a colour for other people, the kind who never spill anything, and it took me a long while to work out that the trick isn't being careful, it's buying one that can take a hot wash without going see through or grey at the seams.
The difference between a good one and a sad one is mostly in your hands before you've even tried it on. Pick it up and it should have a bit of weight to it, not flop like tissue paper, because a top that's too light goes sheer the second you move under any kind of light, shop or daylight, doesn't matter. Run a finger along the seams, they should be flat and close, not puckered, and if there are buttons they want to feel like they'll survive a few years of being done up in a hurry, not the flimsy plastic kind that crack the first time you catch one on a coat zip. The hem matters more than people give it credit for, a proper one lies straight and doesn't curl up after a wash, and the neckline should sit where it's cut, not stretch out into a sad wide scoop after a fortnight. Cheap white cotton pills under the arms within a month, good white cotton just gets softer.
Once you've one that's actually white and actually holding its shape, it does an enormous amount of quiet lifting elsewhere. Under a denim jacket it looks like you've made an effort, under a blazer it looks like you're going somewhere important, tucked into jeans with the sleeves pushed up it looks like you got dressed in four minutes and happened to get it right. It sits under slip dresses in fallish weather too, poking out at the neck and cuff, which is a trick I only learned from watching what the younger ones in the shop wear on a Saturday.
Hold it up to the light before you buy it. Can you see your hand through it, and if you can, do you actually care?