The thing I reach for first is whatever's hanging nearest the door on a morning when I've got three meetings, a nursery run for a friend's child I'm picking up, and no real idea what the weather's doing. A coat or jacket is the one item that has to work before you've even decided what you're wearing underneath it, which is why most of us own too many and still reach for the same two.
Stick to colours that don't ask questions. Navy, charcoal, a proper black, camel if you want something that photographs well against denim, and a dark green that goes with almost everything you already own but somehow still looks like an effort. Bright colours and bold checks are lovely in the shop and then sit at the back of the rail by February, because they only go with the three things you bought to match them.
For pairing, think about what's already in your week rather than what's in a catalogue. A neutral coat goes over a jumper and jeans for the school run, over a shirt and trousers for work, over a dress when you can't be bothered to think about tights. A gilet sits well over a fine knit when a full coat is too much and a cardigan isn't enough. Fleeces and lighter jackets are for the in between days, the ones where you leave the house in sunshine and come back in drizzle, and you'll wear that piece far more than the smart coat you saved up for.
Fit is where most of these go wrong, and it's rarely about size, it's about the shoulder seam. If that sits even a centimetre past where your shoulder actually ends, the whole thing hangs oddly no matter what size label is in it. Check you can cross your arms and reach forward without the back pulling tight across the shoulder blades, because that's the movement you'll make getting into a car or lifting a bag onto a shelf. With coats, leave enough room to get a jumper underneath without the sleeves going short. With anything structured, try sitting down in it before you buy it, not just standing in front of the mirror.
If you've got nothing decent in this department, start with a coat in a plain dark colour that falls somewhere between the knee and mid thigh, long enough to cover a jumper, short enough not to wrap round your legs getting out of a car. Everything else, the gilets, the lighter jackets, the parka for standing at the edge of a football pitch, can come later once you know what you actually do most days.
A good coat has done more for my mornings than any other single item I own.