Baseboards do two jobs. They hide the expansion gap that floating floors and hardwood need at the walls, and they frame the room visually where the wall meets the floor.
Height is the first choice. Taller baseboards suit rooms with higher ceilings and give a more finished, custom feel. Shorter boards work in smaller rooms and with standard ceiling heights. A common guideline is to scale baseboard height with ceiling height.
Profile is the second choice. Simple flat or square-edge boards suit modern homes. Boards with a routed top edge or classic ogee detail fit traditional interiors. Matching your door casing style keeps everything consistent.
Installation is where quality shows. Inside corners should be coped or tightly mitered, outside corners closed cleanly, nail holes filled, and the top edge caulked to the wall.
Replacing baseboards at the same time as a new floor is the most efficient approach, but new baseboards alone can also refresh a room quickly.