The thing about cotton joggers that nobody mentions until you've had a pair for a year is what happens to the knee. Not the waistband, not the hem, the knee, right where it bends when you sit cross legged on a floor or curl up on a sofa. Good ones keep their shape there. Cheap ones go baggy and stay baggy, and you end up with two soft pouches that make you look like you're wearing someone else's trousers even when you're not.
Getting the fit right starts at the waist, not the leg. You want a drawstring that actually does something, not a decorative loop stitched on for show, because that's what stops the whole thing slipping down over the course of a day. On the leg, go for a cut with some taper rather than a straight drop, otherwise the fabric pools around the ankle and catches under boots. If you're between sizes, size down rather than up. A roomy waist you can cinch in is fine, but excess fabric through the thigh and shin just reads as sloppy rather than relaxed, and that's the fit problem that catches most people out. For everyday wear, these sit well with a plain crew neck jumper and trainers for the school run or the supermarket, or under a longline coat with boots when it's cold enough that you need the layer but can't be bothered with real trousers. They also do a quiet job under a blazer for anyone working from home, which is not a combination that photographs well but is extremely common. Wash them inside out on a cool cycle and they'll keep their colour far longer, and resist the tumble dryer unless you enjoy buying the same joggers twice a year, because heat is what kills the cotton's structure and makes the fabric go thin and papery at the knee and seat. Left to dry flat or on a line, a decent pair will go soft without going flimsy, which is the whole point of cotton over anything synthetic. On colour, grey marl is the easiest thing to own because it hides bobbling and doesn't show up every crease, though it will show chalk and flour if you're anywhere near a kitchen or a kid doing crafts. Navy and a soft black are close behind for the same reason, and both go with almost anything you already have on a hanger. Pale colours look lovely for about three wears and then show every mark, so unless you're prepared to wash them constantly, I'd leave them on the rail. A proper charcoal, slightly heathered rather than flat, ages the best of all and still looks deliberate a year in rather than tired.