The real tell on a midi skirt isn't how it looks in the mirror on day one, it's what happens at the back of the waistband after about six weeks of being sat down in all day. Cheap stuff pills and gapes there first, right where nobody checks before they buy.
If you've got none in your rail at all, start with something in a stable cotton or a cotton and viscose mix, A line rather than pencil, hitting somewhere between mid calf and ankle depending on your height, because that shape forgives the most body types and the most days, sitting, cycling, standing on a train, without pulling tight across the thighs or riding up when you walk. A pencil midi is a lovely thing but it's the second skirt, not the first, because it punishes an inch of weight change either way and most of us have an inch of weight change most months. Once you've got the A line sorted you'll know which direction you actually want to go in rather than guessing.
Wear and ageing is where the difference between a good one and a bad one really shows itself, not on the first wear but the fifteenth. A well made skirt in a decent weight of fabric keeps its shape through a full day, doesn't cling and ride when you sit for two hours, and comes out of a cool wash looking roughly like it went in. A cheap one goes thin and see through at the front where your thighs sit against it, the hem drops unevenly because the lining was cut on a different grain to the outer skirt, and the zip starts catching within a season because it's a flimsy nylon one rather than a proper metal tooth. Check the waistband seam and the zip tape with your fingers before you buy if you can, a scratchy, puckered seam there is doing the same thing to your skin every day you wear it. Hang midi skirts rather than folding them over a hanger bar, which stretches a crease into the front that never quite drops out, and wash on a cool cycle inside out if there's any print or colour you care about, because midi lengths get more friction against car seats and chair edges than shorter skirts ever do.
Colour wise, the ones that get worn into the ground rather than left at the back are the quieter shades, a proper navy, a soft olive, a mid grey that doesn't read as office beige, because they go with nearly every top you already own and they don't date the way a very specific print does after one season. Black is the obvious safe choice but it shows chalky marks and fading faster than people expect, so a dark navy often ages better and still looks nearly as sharp against a white shirt.