Lucario! ^^
Lucario is one of my favorite Pokemon (probably somewhere in the top 20) and has served as a reliable partner for me on PBR. He is a bit overrated, but for good reason. Lucario is a very unpredictable Pokemon, having so many options to work with. He can run physical or special, having base 110 Attack and base 115 Sp. Attack. His typing of Steel/Fighting is shared only by Colbalion, and it gives him many resistances, including a x4 resistance to Rock and Dark. This also means he takes almost nothing from the omnipresent Stealth Rock. He is also a great user of priority, packing four priority move of different types; these make up for his below-ideal Speed. Unfortunately, Lucario's below-average defenses mean he won't be taking many hits, and he is also vulnerable to common Fighting, Fire, and Ground attacks. But if you need a Fighting type with many resistances that can get the jump on an unprepared opponent, Lucario is the perfect Pokemon for you.
Recommended tier: OU
Inner Focus prevents flinching. This is inferior to Steadfast in most cases.
Steadfast raises Lucario's Speed one level when he flinches. Flinches are rare outside of Fake Out, but this ability does give Lucario a much-needed Speed boost when it kicks in.
Justified raises Lucario's Attack one level when he gets hit by a Dark attack. Combined with a partner's Beat Up, this can cause Lucario's Attack to reach insanely high levels in Doubles and Triples. You can also switch Lucario into a Dark attack, such as Crunch, though it is risky.
As I said before, Lucario gets four priority moves: Quick Attack, Extremespeed, Bullet Punch, and Vacuum Wave. Extremespeed is usually the best choice, because of its higher priority. Besides, Scizor is superior as a Bullet Punch spammer. If you run special, choose Vacuum Wave. I wouldn't go for Quick Attack, simply because Extremespeed is stronger and has higher priority. Hi Jump Kick is Lucario's strongest attack, but if you don't want to risk the nasty recoil, Close Combat is a safer option at a sacrifice of only 10 base power. Ice Punch decimates common Dragon types. Earthquake is an option, but Lucario's Fighting attacks are usually superior. Thunderpunch is mostly for Gyarados. Stone Edge slays Flying types, but the shaky 80 accuracy can be a problem. Crunch gives Lucario near-perfect coverage combined with his Fighting attacks, and the possible Defense drop is useful, too. Shadow Claw serves the same purpose, though it has an increased crit chance. Lucario's typing means he takes no Sandstorm damage, meaning he's the best user of the Endure/Reversal combo. There's also a bunch of other moves that I'm too lazy to list.
On the special side, Lucario gets the awesome Aura Sphere-possibly the best special Fighting move out there. Dark Pulse and Dragon Pulse usually serve as coverage. Shadow Ball can substitute for Dark Pulse. Lucario also has many boosting moves, including Swords Dance, Bulk Up, Nasty Plot, Calm Mind, Agility, and Hone Claws.
Double and Triple options: There's the Beat Up/Justified combo. Lucario also gets Quick Guard, Heal Pulse, and Follow Me, though he's a bit too frail to really use them.
Countering Lucario: This takes good prediction. Many of Lucario's sets give it perfect or near-perfect coverage. However, you can never know what set its running until it attacks. This is part of what makes it so deadly. Blissey and Chansey won't like Aura Sphere too much, but they can still take the special set pretty well. They can also cripple him with Thunder Wave. Ghosts have to watch out for Crunch, but they can Burn Lucario. Revenge killing is an option, as Lucario's Speed is only base 90.