Matt. Flat matt on a ceiling, and don't overthink it.
The reason is light. A ceiling sits at an angle to your windows and your light fittings, so it catches light in a glancing, raking way that walls never do. Any sheen — even a low-sheen — picks up that raking light and shows up every roller stipple, every lap mark where you let an edge dry, and every lump and dip in the plaster. A dead-flat finish scatters the light instead of reflecting it, so the surface reads smooth and even.
For most ceilings, a dedicated flat matt emulsion is the job. Dulux Trade Vinyl Matt or Crown Trade Matt in brilliant white are the workhorses — cheap, high-coverage, dries flat, and you'll get a ceiling done in two coats. If you want something with a bit more depth than builders' white, Farrow & Ball ceilings often get done in a soft off-white like Wevet or Strong White, and their Estate Emulsion is about as flat as paint comes. Little Greene Intelligent Matt is another lovely flat finish if you're already buying their colours for the walls.
The one thing people ask: *should I colour-match the ceiling to the walls?* For a small or low room, painting the ceiling the same colour as the walls (or a touch lighter) makes the boundary disappear and the space feel taller — Pointing or Slipper Satin work beautifully for this softly-white approach. Bright white ceilings can look stark and draw the eye, so a warm off-white is often the smarter pick.
Two exceptions to the matt rule. Bathrooms and kitchens with steam want a moisture-resistant matt — Dulux Trade Diamond Matt or Earthborn Claypaint both cope with damp better than standard emulsion. And tongue-and-groove or timber ceilings want an eggshell or satinwood for wipeability, not emulsion.
One practical tip: never stop mid-ceiling. Keep a wet edge and work in one continuous run from window-wall outward, so the light never catches a dried join.