Simply insane

First off we have the classic standard choice specs Slaking. This set is very self explanatory.

Then we have the good old bulk up slack off Slaking, Slaking sadly doesn't get calm mind, I guess that would push him to Ubers though. Sucker punch is used here for those nasty ghost types.

Then we have the focus energy crit Slaking, you might be saying: "you only need a crit stage of 3 to have guaranteed crits." Then I'll say: "Yes but Slaking is such a beast that a crit stage of 3 doesn't do him justice, you need 4."

Then we have the fling facade flame orb Slaking, this set is honestly so broken, you can literally just get +1 attack, a boosted facade and your enemy burned in only 6 turns! I mean how insanely good is that?

Next up there's the sunny day Slaking. Sunny day makes Slaking's fire blasts and solar beams rise up to extreme power. In the sun Slaking will only need 2 turns for every solar beam instead of the usual 2 turns. Simply insane.

Finally we have hydrogen bomb Slaking. You nuke 1 thing but then you can't do anything anymore because you've blown up the entire world.