Wall for Terlor (page 6)

Just because a Pokemon is the best one on your team doesn't mean it's the best one in the whole game.
I'll compare Slaking to a Zangoose by giving both 160000 experience and using them against Steven's Claydol.

In Ruby:
84 Atk Slaking Return vs. 0 HP / 0 Def Claydol: 77-91 (52 - 61.4%) -- guaranteed 2HKO
84 Atk Zangoose Return vs. 0 HP / 0 Def Claydol: 69-82 (46.6 - 55.4%) -- 71.1% chance to 2HKO
In Omega Ruby:
84 Atk Slaking Return vs. 0 HP / 0 Def Claydol: 76-91 (51.3 - 61.4%) -- guaranteed 2HKO
84 Atk Zangoose Return vs. 0 HP / 0 Def Claydol: 69-82 (46.6 - 55.4%) -- 71.9% chance to 2HKO

When they're that close in terms of attack power, the one with swords dance and not truant is better.