Highlighted weapons are free options.
Bennett's main utility as a support comes from the huge Flat ATK buff his Burst provides. This buff only scales with his Base Attack, so it is unaffected by any external ATK buff.
Bennett's C6 infuses melee weapons with Pyro, which can mess up a few teams. However, if you have C6 Bennett, you more than likely have some of the supports those teams prefer over Bennett anyway.
After reaching Energy Recharge requirements, Bennett can be optimized for his personal damage or for his healing. Choose the matching main stats and substats if you want to do so.