RIOLU - THE COPYCAT SWEEPER
Some people may be thinking: 'Why the hell has some idiot gone and done some thread on Riolu in OU?"
If you are in this bracket, I urge you to watch this video.
http://www.youtube.com/watch?v=-KId0j4g5KE
Have you watched? If you haven't, WHY NOT??? Go watch it.
If you have watched it, you're probably intrigued. You probably know that people have been using Lv1 Aron on Sandstorm teams to abuse Sturdy+Endeavor to relative success, as if your team lacks something that can stop it (i.e something that doesn't take damage from sandstorm), you're basically screwed. However, Riolu is more obscure. The tactic it uses is known as 'speed shuffling', which involves phazing moves bypassing their negative priority, and abusing this to rack up residual damage from Stealth Rock and if possible, Spikes. There are various ways of performing 'speed shuffling' - the most common one is with a fast Sleep Talker. However, some people have based whole teams round speed shuffling, having one pokemon to use Stealth Rock, then smacking on a Pokemon like Skarmory that pack only moves like Roar and Whirlwind, and the rest of the team is based up of assist users, normally choice scarfed unless it's Liepard, who also has Prankster. This way, the Assist user goes first, generates a phazing move, and forces out the opponent, racking up residual damage. They then have a dedicated finisher pokemon who cleans up the last pokemon (who cannot be phazed out), normally with just one move. Riolu, however, has developed a new form of Speed shuffling, involving a phazing move and a priority Copycat.
SET: Speed Shuffler
Riolu
-Roar
-Circle Throw
-Copycat
-filler
Nature: Jolly
EVs: 252Atk/252Spe
Item: Focus Sash
Ability: Prankster
So, what makes it work? Well, Riolu obtained the new ability Prankster from the Dream World. In case you didn't know, this ability increases the priority of non-attacking moves by one stage. The idea is to set up Stealth Rock, and if possible, Spikes (although Spikes are not necessary), while keeping them off your side of the field to preserve Riolu's Focus Sash. Then send in Riolu when one of your team members faints, survive the hit with the sash, and use a phazing move (roar if the opponent has a Ghost type on his team). This will drag out the next Pokemon. Then, Riolu can spam a priority Copycat to copy HIS OWN MOVE and bypass the negative priority of his phazing move. This allows Riolu to speed shuffle and rack up hazard damage. Copycat has 20PP, 32 with PP max. 32 turns of shuffling the opponent's team and hazard damage! Hell yeah. Riolu will cause more ragequits than any other pokemon (hopefully... imagine their face when they realise they're being owned by a Riolu, of all Pokemon... :-D)
In case you're sceptical of Copycat, a) Watch that goddamn video, and b) Here's the description from Psypoke:
So what stops Riolu sweeping?
Unfortunately, many things, which means if you want your Riolu to sweep, you'll have to provide the necessary team support.
1) LACK OF ENTRY HAZARDS ON THE OPPONENT'S SIDE. Yeah, if there's no Stealth Rock up, you aren't doing any damage when you phaze the opponent. That sucks. YOU MUST HAVE AT LEAST STEALTH ROCK UP.
2) ENTRY HAZARDS ON YOUR SIDE. This will break Riolu's focus sash and as a result ruin the strategy. To counter this you could use a Magic Bounce user or a Rapid Spinner to keep off hazards.
3) FASTER PRIORITY USERS. This means any Pokemon packing ExtremeSpeed, as well as priority users who invest fully in Speed with over 60 base, which outspeeds Riolu. With team preview, MAKE SURE YOU NOTE ANY POTENTIAL PRIORITY USERS. Then proceed to eliminate them accordingly.
4) SANDSTORM OR HAIL. Unfortunately Riolu does not share the steel typing that its evolution Lucario has; this means that Sandstorm will negate its Focus Sash at worst, or reduce its 'sweep' to 16 turns at best. To stop this, you could put Drought Ninetales or Drizzle Politoed on the team, or run Sunny Day/Rain Dance on one or more of your pokemon.
So yeah. You can now see that with the appropriate team support Riolu can actually be threatening in OU. Gimmicky, sure, but also effective. Also, I take no credit for the set - I DID NOT INVENT IT. Whoever did is a genius.

Some people may be thinking: 'Why the hell has some idiot gone and done some thread on Riolu in OU?"
If you are in this bracket, I urge you to watch this video.
http://www.youtube.com/watch?v=-KId0j4g5KE
Have you watched? If you haven't, WHY NOT??? Go watch it.
If you have watched it, you're probably intrigued. You probably know that people have been using Lv1 Aron on Sandstorm teams to abuse Sturdy+Endeavor to relative success, as if your team lacks something that can stop it (i.e something that doesn't take damage from sandstorm), you're basically screwed. However, Riolu is more obscure. The tactic it uses is known as 'speed shuffling', which involves phazing moves bypassing their negative priority, and abusing this to rack up residual damage from Stealth Rock and if possible, Spikes. There are various ways of performing 'speed shuffling' - the most common one is with a fast Sleep Talker. However, some people have based whole teams round speed shuffling, having one pokemon to use Stealth Rock, then smacking on a Pokemon like Skarmory that pack only moves like Roar and Whirlwind, and the rest of the team is based up of assist users, normally choice scarfed unless it's Liepard, who also has Prankster. This way, the Assist user goes first, generates a phazing move, and forces out the opponent, racking up residual damage. They then have a dedicated finisher pokemon who cleans up the last pokemon (who cannot be phazed out), normally with just one move. Riolu, however, has developed a new form of Speed shuffling, involving a phazing move and a priority Copycat.
SET: Speed Shuffler
Riolu
-Roar
-Circle Throw
-Copycat
-filler
Nature: Jolly
EVs: 252Atk/252Spe
Item: Focus Sash
Ability: Prankster
So, what makes it work? Well, Riolu obtained the new ability Prankster from the Dream World. In case you didn't know, this ability increases the priority of non-attacking moves by one stage. The idea is to set up Stealth Rock, and if possible, Spikes (although Spikes are not necessary), while keeping them off your side of the field to preserve Riolu's Focus Sash. Then send in Riolu when one of your team members faints, survive the hit with the sash, and use a phazing move (roar if the opponent has a Ghost type on his team). This will drag out the next Pokemon. Then, Riolu can spam a priority Copycat to copy HIS OWN MOVE and bypass the negative priority of his phazing move. This allows Riolu to speed shuffle and rack up hazard damage. Copycat has 20PP, 32 with PP max. 32 turns of shuffling the opponent's team and hazard damage! Hell yeah. Riolu will cause more ragequits than any other pokemon (hopefully... imagine their face when they realise they're being owned by a Riolu, of all Pokemon... :-D)
In case you're sceptical of Copycat, a) Watch that goddamn video, and b) Here's the description from Psypoke:
Because your phazing move has negative priority, you will always go last, barring trick room (which still can't stop the strategy because the user will be forced out before it can move, meaning the phazing move will still be the last move used). This means that when Riolu uses his priority Copycat, barring faster priority moves, he will copy the his phazing move. This cycle continues...The user performs the battle's last successful move. This move could be from the current opponent, a previous opponent, a teammate or even another move from the user's own moveset. This move fails if no Pokemon has used a move yet. Copycat cannot copy itself.
So what stops Riolu sweeping?
Unfortunately, many things, which means if you want your Riolu to sweep, you'll have to provide the necessary team support.
1) LACK OF ENTRY HAZARDS ON THE OPPONENT'S SIDE. Yeah, if there's no Stealth Rock up, you aren't doing any damage when you phaze the opponent. That sucks. YOU MUST HAVE AT LEAST STEALTH ROCK UP.
2) ENTRY HAZARDS ON YOUR SIDE. This will break Riolu's focus sash and as a result ruin the strategy. To counter this you could use a Magic Bounce user or a Rapid Spinner to keep off hazards.
3) FASTER PRIORITY USERS. This means any Pokemon packing ExtremeSpeed, as well as priority users who invest fully in Speed with over 60 base, which outspeeds Riolu. With team preview, MAKE SURE YOU NOTE ANY POTENTIAL PRIORITY USERS. Then proceed to eliminate them accordingly.
4) SANDSTORM OR HAIL. Unfortunately Riolu does not share the steel typing that its evolution Lucario has; this means that Sandstorm will negate its Focus Sash at worst, or reduce its 'sweep' to 16 turns at best. To stop this, you could put Drought Ninetales or Drizzle Politoed on the team, or run Sunny Day/Rain Dance on one or more of your pokemon.
So yeah. You can now see that with the appropriate team support Riolu can actually be threatening in OU. Gimmicky, sure, but also effective. Also, I take no credit for the set - I DID NOT INVENT IT. Whoever did is a genius.