You can spot a good fleece from the other side of a room. The good one sits with a bit of structure, a proper density to it, a collar that stands up on its own and a zip that doesn't bite. The bad one is limp before you've even put it on, see-through under bright light, and by the second wash it's gone bobbly across the chest like it's been dragged through a hedge. Hold both in your hands and you already know which is coming home with you.
It's not a piece that wants company from anything else sporty. Two technical fabrics together, a fleece over a running top under a gilet, and you've accidentally dressed for a hike nobody invited you on. It also sits oddly against anything slippery, a satin skirt or a silky shirt, the fleece grips and bunches where it should skim. And there are days it's simply the wrong call. It's not made for proper weather, a downpour will soak straight through it, and it's not a smart option, however neat the zip, it still reads as off duty the second you need to look like you tried. Trying to wear one under a fitted coat is a losing fight too, the bulk just sits wrong at the shoulders and ruins the line of both pieces.
If you own none, start with something mid weight, not the thin fleece that feels like a blanket from a budget flight, not the thick teddy texture either, that one's lovely but bulky and better as a second buy once you know you'll wear the first. A simple zip through with a funnel collar and a proper two way zip gives you the most use, layers under a coat without a fight, works alone on a mild day, and doesn't commit you to a particular sporty look the way a half zip pullover style can.
Colour wise, keep it plain and you'll wear it for years. Navy, stone, a soft grey, all of them sit quietly under things and never argue with whatever's underneath. Bright primary colours and anything neon date fast and only really work on their own, which limits you more than you'd want. An oatmeal or putty shade in that teddy texture looks expensive even when it wasn't. Thrown over the jeans you already reach for without thinking, it just makes the whole thing look like you planned it.