Stats
160 Atk is the figurehead here, one of the highest attack stats on a non legend/mega. 150 HP is awesome as well. 100 spd is surprising, and 100 def is good. 65 SpDef is awful though. SpAtk? Who cares? Anyone who runs special Slaking is evidently rather stupid.
Ability
Truant is awful, but you're stuck with it. Something this powerful is evidently going to have a catch somewhere, so look no further. It's here.
Movepool
Physically, it gets Return, Facade, Giga Impact, Gunk Shot, the elemental punches, Sucker Punch, Shadow Claw, EQ, Rock Slide and Hammer Arm. Awesome, but fairly limited. Setup-wise, Bulk Up and Slack Off are the main things here. Heal and Atk/Def boost will be a cause for concern, but Truant is a right pain in the backside.
Set
Lord of the Sloths
Slaking@Life Orb
252 Atk/252 HP/4 Spd
-Giga Impact
-Sucker Punch
-One of the elemental punches
-EQ/Rock Slide/Shadow Claw/Slack Off
Simple enough set. Giga Impact is awesome due to the fact that you'll miss the turn due to Truant. The rest just allow for a coverage festival, or healing.
Doubles/Triples
Slaking is extremely viable here, especially with Cherrim support. Anything who can get rid of its ability is a huge boost, not to mention a cause for ragequits. Just run the right set, and you'll be A-Inkay.
Slaking-Kong
Slaking@Life Orb
252 HP/252 Atk/4 Spd
-Bulk Up
-Return
-Sucker Punch
-Slack Off
OK, ability switch early on, and then Bulk Up and hit anything hard. Slack Off the damage for LO and any surviving enemies that hit you. Slaking without Truant is a sweeper of such power, nothing can really handle it. Six pokemon hard wall this set, namely Lucario, Cobalion, Carbink, Tyranitar, Bisharp and Mawile. Anything else gets hit neutral by that Atk, or Ghost/Psychic gets SE from Sucker Punch.
Counters
Anything Special based will hurt the King hard. Also, be wary of that speed, it could ruin you. Aside from special, Fighting type moves are the way to go. Focus Blast/Aura Sphere will probably one-shot it. Mega Mewtwo will kill him, provided Sucker Punch doesn't OHKO. Lucario hits hard as well, but Fire Punch and EQ will devastate it.
Pros/Cons
+Awesome Atk/Def/HP/Spd
+Decent physical movepool
+It gets Bulk Up
-Awful SpDef