Leader Dump:
Druddigon:
Abilities: Rough Skin, Sheer Force, Mold Breaker.
Physical Moves: Crunch, Dragon Claw, Dragon Tail, Super Power, Outrage, Crush Claw, Iron Tail, Pursuit, Sucker Punch, Aqua Tail, Fire Punch, Gunk Shot, Thunder Punch, Iron Head, Earthquake, Return, Shadow Claw, Rock Slide.
Special Moves: Draco Meteor, Dragon Pulse, Heat Wave, Shock Wave, Surf, Hidden Power, Flamethrower, Sludge Bomb, Focus Blast, Flash Cannon, Dark Pulse.
Status Moves: Hone Claws, Glare, Endure, Stealth Rock, Roar, Toxic, Taunt.
Amazing Leader for offense. If Garchomp has taught us anything it's that Rough Skin + Rocky Helmet with SR and Dragon Tail makes for one epic SR setter and chip damage for offensive teams. Back up this chip damage with some powerful Sheer Force boosted special (and physical) attacks, and a Hone Claws + Mold Breaker physical attacker to muscle past Unaware teams, and you've got yourself some pretty solid possibilities.
Barbaracle:
Abilities: Sniper, Tough Claws, Pickpocket.
Physical Moves: Skull Bash, Stone Edge, Night Slash, Razor Shell, Cross Chop, Dual Chop, Low Kick, Super Power, Dragon Claw, Earthquake, Return, Aerial Ace, Shadow Claw, X-Scissor, Poison Jab.
Special Moves: Ancient Power, Earth Power, Surf, Hidden Power, Ice Beam, Sludge Wave, Focus Blast, Scald, Grass Knot.
Status Moves: Shell Smash, Rock Polish, Swords Dance, Hone Claws, Bulk Up, Switcheroo, Stealth Rock, Toxic, Taunt.
LOTs of boosting options, mostly for physical attackers but Special Attackers can still destroys with a SS and each option has its merits. Not many defensive options, but hopefully the combination of SR, Boosting, great coverage, and powerful Tough Claws/Sniper boosted attacks (Don't forget Grass Knot!) should be enough to dismantle most teams. Also not sure how well this'd go in practice, but a cool idea for a partner could be Snorlax with Tough Claws Skull Bash and Shadow Claw + Rest Talk.
Dunsparce:
Abilities: Run Away, Serene Grace, Rattled.
Physical Moves: Pursuit, Double Edge, Drill Run, Body Slam, Astonish, Bite, Headbutt, Zen Headbutt, Rock Slide, Aqua Tail, Iron Tail, Earthquake, Gyro Ball, Poison Jab, Wild Charge.
Special Moves: Ancient Power, Hex, Water Pulse, Ice Beam, Thunderbolt, Thunder, Flame Thrower, Fire Blast, Shadow Ball.
Status Moves: Yawn, Roost, Glare, Coil, Agility, Stealth Rock, Calm Mind.
BAN DUNSPARCE. Never imagined myself saying that but I do indeed believe that Dunsparce (or more so Serene Grace) deserves to be looked into. Body Slam has a 60% chance to paralyze, Thunder has a 60% chance to paralyze, Glare has a 100% chance to paralyze even Ground types. Astonish, Bite, Headbutt and Rock Slide all have a 60% chance to flinch, Zen Headbutt has 40% chance to flinch. In case you haven't quite gotten it yet that is a fuck tonne of options for para flinch. Not to mention HEX. I mean a Dunsparce team is obviously gonna aim to just paralyze your entire team and go to town with Paraflinch, but they also have the option to absolutely Nuke your crippled ass shit with Hex. Also Water Pulse has a 40% chance to confuse. Paraflinchfusion anyone? Your electric types aren't exactly safe either with Dunsparce having access to EQ and Pursuit (could a specialised Ele-Killer Krook perhaps work?).
And that's just talking paraflinch shenanigans. Let's not ignore the power of Coil/Calm Mind + Roost. Dunsparce also has access to SR. Iron Tail has a 60% chance to lower your defence, Shadow Ball has a 40% chance to lower your special defence, Poison Jab has a 60% chance to poison. And just in general Dunsparce has some really nice coverage. Also don't underestimate Rattled as an option.
And just lastly, however I do agree that lower tier mons are some of the best leaders (I mean look at the mons I've discussed in this post), I also think that it's just more fun to find niches for lower tier pokemon, and that many of the best leaders are still OU mons and that their are a lot of good OU leaders that people haven't bothered to explore yet.
I mean people have talked about Sableye and Clefable, but I'm sure there are many more.
Take looks at Celebi and Infernape for instance. Both have SR and Recovery. Physical and Special boosting moves. Priority moves. Solid Physical and Special coverage. U-Turn and/or Baton Pass. Reliable status inflicting moves (Thunder Wave and Will-O-Wisp respectively). Plenty of other utility moves. And at least one useful ability.
Also Jirachi is good. But. Serene. Grace. Is. Aids.