Least rigid maxillary major connector is:

U -Shaped Palatal Connector: From both the patient's standpoint and a mechanical standpoint, the U-shaped palatal connector is  the least desirable of maxillary major connectors. It should never be used arbitrarily. When a large  inoperable palatal torus exists and occasionally when several anterior teeth are to be replaced, the U-  shaped palatal connector may have to be used. In most instances, however, other designs will serve  more effectively. The following are the principal objections to the use of the U-shaped connector: Its lack of rigidity (compared with other designs) can allow lateral flexure under occlusal forces,  which may induce torque or direct lateral force to abutment teeth. The design fails to provide good support characteristics and may permit impingement of  underlying tissue when subjected to occlusal loading. Bulk to enhance rigidity results in increased thickness in areas that are a hindrance to the tongue.