A beanie is the one hat I never have to talk myself into, because it doesn't ask anything of your face shape or your hair or how brave you're feeling that morning, it just goes on and gets the job done.
The colours that make a beanie worth having are the quiet ones, oatmeal, charcoal, a proper navy, camel if you want something a shade warmer against the skin. Bright colours look lovely on the hanger and then you realise you own exactly one coat they go with. A fine merino knit sits close to the head and doesn't make you look like you're wearing a tea cosy, while a chunkier rib has more presence under a hood or against a bigger coat collar. Acrylic is the one to be wary of, it keeps you warm but it can feel scratchy against the forehead after an hour, and it pills faster than wool ever will. A cashmere blend is lovely if you're willing to treat it gently, but a good merino does nearly as much for considerably less fuss.
A beanie earns its place against things you already own rather than anything new. It sits well under a parka hood without going flat, it looks right with a denim jacket on a cold bright morning, and it does something for a plain wool coat that a scarf alone can't manage, because it finishes the whole silhouette rather than just warming your neck. With hair down it softens the line at your forehead, with a ponytail or hair tucked away it reads cleaner and a bit more deliberate. Over months, the main thing that happens to a beanie is pilling, especially where it rubs against a coat collar on the bus or gets stuffed into a bag pocket rather than folded. Wool copes with this better than you'd expect if you wash it on a cool wool cycle or by hand, then lay it flat to dry rather than hanging it, which stretches the brim out of shape. A fabric comb or a cheap fabric shaver sorts out the bobbling in a couple of minutes and the hat looks new again, which feels like cheating.
If you've got none, start with a plain ribbed merino in a neutral colour with a folded brim and no pompom, it works with everything you own and nothing about it dates.
Tomorrow morning I'd put mine on with a grey wool coat, a plain jumper underneath, and the kind of jeans that have gone soft from being worn too often.