I believe that the best competitive Riolu set would be the infamous 'Copycat Roar' set in which Riolu exploits his access to Copycat, a phazing move and Prankster in order to shuffle Pokemon and rack up entry hazard damage.
Riolu @ Eviolite
EVs: 248 HP / 112 Def / 52 Spd / 96 SDef
- Drain Punch
While it may seem enticing at first glance, an offensive set in LC is generally outclassed and it ends up abusing Protect + Copycat regardless.