Lucario for B/B+
Now you may be wondering "Why is Lucario even being mentioned, OU already has some great Fighting types" well, Lucario is Pokemon that can help many weaknesses, acting as a glue for other teams to use. For one, Lucario has some great offensive stats, 110 base attack and 115 base special attack, and access to
both Swords Dance and Nasty Plot, A nice physical move pool, but a lacking special movepool, but what it has in its special move pool is good, having access to Dragon Pulse, Aura Sphere, and Flash Cannon, it can hurt alot of pokemon with a nasty plot boost (see calcs below) and with swords dance and a very good Physical movepool, it can pack a punch for any team, its only problems are Skarmory, Bulky Ground, Psychic, Fairy types and Gyarados (not mega). those threats are gone if you have a boost (well not gyarados)
Calcs (physical luke):
Clefable
252+ Atk Life Orb Lucario Iron Tail vs. 252 HP / 160 Def Clefable: 439-523 (111.4 - 132.7%) -- guaranteed OHKO
252+ Atk Life Orb Lucario Iron Tail vs. 252 HP / 252+ Def Clefable: 367-432 (93.1 - 109.6%) -- 56.3% chance to OHKO
+2 252+ Atk Life Orb Lucario Iron Tail vs. 252 HP / 160 Def Clefable: 881-1037 (223.6 - 263.1%) -- guaranteed OHKO
Defensive Landorus
-1 252+ Atk Life Orb Lucario Extreme Speed vs. 252 HP / 240+ Def Landorus-T: 58-69 (15.1 - 18%) -- possible 8HKO after Leftovers recovery
-1 252+ Atk Life Orb Lucario Iron Tail vs. 252 HP / 240+ Def Landorus-T: 109-129 (28.5 - 33.7%) -- 95.9% chance to 4HKO after Leftovers recovery
-1 252+ Atk Life Orb Lucario Close Combat vs. 252 HP / 240+ Def Landorus-T: 65-77 (17 - 20.1%) -- possible 7HKO after Leftovers recovery
+1 252+ Atk Life Orb Lucario Iron Tail vs. 252 HP / 240+ Def Landorus-T: 242-286 (63.3 - 74.8%) -- guaranteed 2HKO after Leftovers recovery
+1 252+ Atk Life Orb Lucario Extreme Speed vs. 252 HP / 240+ Def Landorus-T: 130-153 (34 - 40%) -- 38.6% chance to 3HKO after Leftovers recovery
+1 252+ Atk Life Orb Lucario Close Combat vs. 252 HP / 240+ Def Landorus-T: 144-172 (37.6 - 45%) -- guaranteed 3HKO after Leftovers recovery
Defensive Mew
252+ Atk Life Orb Lucario Close Combat vs. 252 HP / 224+ Def Mew: 91-108 (22.5 - 26.7%) -- possible 5HKO after Leftovers recovery
252+ Atk Life Orb Lucario Extreme Speed vs. 252 HP / 224+ Def Mew: 82-97 (20.2 - 24%) -- possible 6HKO after Leftovers recovery
252+ Atk Life Orb Lucario Iron Tail vs. 252 HP / 224+ Def Mew: 153-181 (37.8 - 44.8%) -- guaranteed 3HKO after Leftovers recovery
+2 252+ Atk Life Orb Lucario Close Combat vs. 252 HP / 224+ Def Mew: 182-214 (45 - 52.9%) -- guaranteed 3HKO after Leftovers recovery
+2 252+ Atk Life Orb Lucario Extreme Speed vs. 252 HP / 224+ Def Mew: 162-192 (40 - 47.5%) -- guaranteed 3HKO after Leftovers recovery
+2 252+ Atk Life Orb Lucario Iron Tail vs. 252 HP / 224+ Def Mew: 305-360 (75.4 - 89.1%) -- guaranteed 2HKO after Leftovers recovery
These pokemon, although not so popular (besides clefable) are some of the only checks or counters to Lucario, although a lot of the Pokemon have some form of a move to defeat Lucario, but it can hurt some Pokemon, without worrying about these 3 (but there are probably more)
Edit: forgot about 1 thing
Lucario can also act as a Ice Type killer, resisting all ice type moves, and hurting both Weavile and Mamoswine with Close Combat and Iron Tail (possibly bullet punch)