This totally depends on the moveset and the kind of Lucario that you are running. If you are running physical, I suggest Stone Edge as it has higher power and a crit chance. You should only pick Rock Slide if you are doing doubles. If you are running mixed or special, choose Flash Cannon. It is a STAB move so it has a power boost and it also takes care of fairies.
Like I said, it depends on the moveset. Also, like SapphireDiancie pointed out, it depends if you will mega-evolve it.