Slaking
HP: 150
ATK: 160
DEF: 100
SpATK: 95
SpDEF: 65
SPEED: 100
Ability: Truant
People look at this gorilla's base stats and they start to freak out. Base 150 HP!? Base 160 Attack!? Base 100 Speed!? Then they look at his ability and toss him in the NU trash. Either with Giga Impact or Retaliate you can cause a lot of havoc in the 5th gen UU metagame against most threats.
Slaking @ Choice Scarf/Choice Band/Leftovers
Ability: Truant
Nature: Jolly/Adamant
- Giga Impact/Retaliate
- Sucker Punch
- Substitute/Substitute/Slack Off/Rest/Counter/Focus Punch
- Slack Off/Substitute/Slack Off/Rest/Counter/Focus Punch
EVs with CS: 252 atk/252 spe/4 HP
EVs with CB/Leftovers: 252 atk/252 HP/ 4 Spe
Focus Punch can be used if are good at predicting switches to steels. Sucker Punch deals with Ghosts. Giga Impact destroys most threats. Substitute, Rest and Slack Off can be used to stall or even tank to a small degree.
If you want to switch out after attacking go with Retaliate over Giga Impact. If you use Retaliate after a teammate is KO'd and you factor in STAB the BP rises to 210. Giga Impact is just 15 base power higher with STAB.
Giga Impact/Retaliate can OHKO most offensive threats that don't resist it. Bisharp for example is a 2HKO if it has 252 in HP. Pokemon like Aerodactyl are OHKO'd however. Retaliate on Slaking is useful because most of the time when a teammate is KO'd it is because of an offensive pokemon unless you threw in death fodder towards a tank.
Slaking can be really useful in specific roles, but he has a few shortcomings. He cannot switch out after using Giga Impact, but he can if he uses Retaliate. His ability also causes him to wait a turn after using any move, which makes it hard to Tank or Stall with him at all. He is also weak to Mach Punch, but outside of OU he doesn't have many powerful mach punch users to be worried about, except for Iron Fist Hitmonchan.
(made changes to remove the mention of being able to switch out after using Giga Impact because it has been proven to be false here: http://www.smogon.com/forums/showpost.php?p=3004772&postcount=1)