For the physical set, Bullet Punch is the best Steel STAB you could ask for, combining both the priority of Extreme Speed and the coverage of Metal Claw. This allows you to drop E-Speed for a coverage move like Earthquake or Ice Punch. Bullet Punch is an Egg move, though.
For the mixed set, Flash Cannon is good, but is kind of pointless because if you're going to be using Swords Dance Bullet Punch is still better. Most mixed Lucario that I've seen use Bullet Punch anyways. Flash Cannon would fit better on a fully special set with Hidden Power [Ice] and Aura Sphere.
Of course, if you don't have access to/don't want to breed Bullet Punch onto a Lucario, then Flash Cannon all the way, or even Iron Tail if you're feeling gutsy :)