pokemon is immensely diverse. I almost got my *** whooped by a Purugly last night....
Some of Smogon's sets are the best. Some of them (Metagross, Gyarados) I've been using for 2 years and have just recently been added as standard sets, so maybe they're catching on.
Anyone with any sense of creativity can usually one-up them though. You just need the right tools. So I'll provide some:
http://www.serebii.net/pokedex-dp/
http://www.smogon.com/dp/
http://www.geocities.com/xactcreations/FixEVs.htm
http://users.smogon.com/X-Act/defense.html
http://www.metalkid.info/Pokemon/OnlineProgram/Calculators/HiddenPower.aspx
http://libelldra.com/competitive/damage/
Ideally, you start by reading Smogon's overview for the pokemon. Smogon is good at showing what a pokemon can do, common counters, and is a generally good place for a starting point.
Let's choose Swampert. Swampert is obviously good on the physical side and will probably be used best fighting other physical pokemon that won't appreciate its water attacks. So let's do that.
Notice immediately that one of the physical pokemon we'll want to beat is Garchomp, as Garchomp fits into that group of pokemon Swampert is better off fighting. To OHKO Garchomp with a non-stab ice beam, we need 269 special attack. That's a big investment that Swampert can't really afford. Since Swampert usually has Leftovers, Choice Band, or a Salac Berry on its standard sets, we can afford to feign one of the two latter and cheat the stats a little bit. Since Life Orb compromises his defensive ability, we'll use Expert belt instead. Now we check the damage calc.
Swampert needs 225 Sp Att (much better) to OHKO Garchomp with ice beam, or 210 with Stealth Rock on the field. Even without SR on the field, it will still OHKO 58% of the time, which is pretty acceptable. 210 SpA is a meager 16 EVs, so we're using it. Since we're using Expert Belt, we can also afford to invest more into defenses. First for EVs I'm going to boost it's attack, since the base 110 will get the largest overall boost, but will also threaten 2HKOs when I can't hit SE for the Expert Belt to kick in. This won't compromise the defenses much, and leaves a higher overall return. The last stat I want to worry about is speed, but at 140 I still want to outspeed min speed regis @ a neutral nature, which is 136. All Perfect 31s on a Brave nature isn't a legit combination. Turns out, the only legal combination to keep Swampert perfect in all non-speed IVs and to pass 136 speed is to make the speed IV 30. Since we don't particularly have anything better to do, we can put in Curse over the filler spot. At least that way, Swampert can curse and be able to OHKO any non-Shuca Metagross.
We finish with this:
Swampert @ Expert Belt
Brave, Torrent
252 HP / 68 Att / 136 Def / 16 SpA / 36 SpD
Earthquake
Waterfall
Ice Beam
Curse