Assuming this is competitive Battle Spot, the best item, both Gen6 and Gen7 is probably Lucarionite. Mega Lucario is great for many reasons. 1. Its ability, Adaptability, gives double the boost to your STAB attacks. Period. 2. It is both a great physical and special sweeper, having access to things like Swords Dance, Close Combat, Bullet Punch, and the elemental punches while it also has moves such as Nasty Plot, Aura Sphere, Flash Cannon, Dark Pulse, and Vacuum Wave.
Life Orb is the next best item because it provides a constant power boost.
Finally, you might want to get a new Lucario that has either the Timid or Jolly Nature. Don't do Brave Nature- it lowers your speed instead of raising it.
If this isn't competitive battling for any reason, don't stress on what item to give it. In-game teams don't require 48+ BP items: an elemental plate, gem, or Z-crystal would be fine.