Where ceiling meets wall.
Use drywall corner bead for corners (embedded metal with tape, without tape, and nylon as well) because you might hit them and this protects the drywall corners. Use seam tape for the ceiling to wall. It will have a split down the middle where you can fold it in half. Cut your strip (I always go slightly longer then trim when the first layer is dry) fold in half, mud, press and smooth. Just like the seams between the drywall panels on the wall or ceiling.