Oh hey this thread!!
I'll post my current PO team here. It's a lot of fun and it does pretty well. This is a slightly revised version... the Umbreon came in to replace a Slowbro that wasn't really doing anything for the team. Now everyone has something to do, and things are going well! I do have a few weaknesses, though, so take a look and tell me what you think
At a glance:
Attachment 10154Attachment 10158Attachment 10155Attachment 10156Attachment 10157Attachment 10159
In-depth:
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Roserade: Lead
Attachment 10154
Calm, Leftovers, Natural Cure
252 HP / 120 Def / 136 SpDef
Spikes
Leech Seed
Giga Drain
Hidden Power Ice
She's a mixed defensive Roserade with an emphasis on special defense. She can generally take a hit defensively, but only one. :P A combination of Leech Seed, Leftovers, and STAB Giga Drain (which actually packs quite a punch) means that she won't go down easily. Hidden Power Ice deals with lead dragons, and Spikes is my team's entry hazard. I typically open with Leech Seed because Espeon is a common switch-in, and as a grass type, she won't get seeded from the Magic Bounce. Then I can switch in to Umbreon and start the battle with an advantage.

If they don't switch in, then I go offensive with Giga Drain, switch out, or start spiking up the battlefield, depending on sort of lead I'm dealing with.
EDIT: I forgot to mention that she also takes care of Toxic Spikes! ;D
A classic dedicated lead, specially defensive Roserade; minus the Sleep Powder. Never found room for one on my own team, but only because I've never cared for Grass types at all (and likewise, Fire-Grass-Water cores).
There is a slight issue I dislike for this set, however: it's the HP Ice. Uninvested, Roserade is just damn slow. How many Dragons have ya actually hard-countered with said HP Ice? Very rarely are Dragons slow unless they're D-nite, and said bulky example happily avoids your Spikes to keep Multiscale workin' and makin' that uninvested HP Ice do laughable damage. One Roost later, and HP Ice does even less. I think Sleep Powder is a much better move to use. A sleep move certainly differentiates Roserade from the premier spiker Ferrothorn, and will shut down said bulky D-nite with ease.
Of course, then enemy Grass types would utterly wall it once they discover you lack Sludge Bomb, so this bulky moveset would have a problem type to deal with either way. If Roserade's HP Ice hasn't been too helpful, do consider replacing it with Sleep Powder.
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Umbreon: Tank/Phazer
Attachment 10158
Impish, Leftovers, Synchronize
252 HP / 52 Def / 204 SpDef
Payback
Curse
Yawn
Moonlight
Umbreon takes a hit like no other. With outstanding HP and special defense, as well as very good defense, it takes QUITE a bit to take him down. After a few curses, Payback becomes immensely powerful, and nothing can significantly hurt him. In the downtime, Yawn forces switches and Moonlight restores HP. Once, he put a Scizor to sleep and got two curses off, which allowed his Payback to do more damage than his Bug Bite, and ultimately win the battle. After setting up, this monster can sweep entire teams -- I've seen it happen all too often.
Poor Umbreon was hit hard this generation. Moonlight is impaired even more often with both Sandstorm and Rain teams in abundance, as well as the fact that there's a fighting type (or two, even) on every bloody team. If it's doing it's job more often than not, I won't argue against it of course.
Also, if weather is seriously hampering Umbreon's day, never forget about Wish. Wish might take more prediction, but it allows Umbreon to be both a sweeper and a team player on top of being weather-proof. With Salamence being weak to SR and Mamoswine being anythin' but bulky, Wish could really help them stay around to fight another day. Since your Umbreon has maximum investments in HP, it's Wish would be especially useful to give to it's team mates.
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Skarmory: Troll
Attachment 10155
Adamant, Leftovers, Weak Armor
252 Atk / 252 Spd / 4 HP
Swords Dance
Drill Peck
Rock Slide
Steel Wing
Here's what happens: I switch this guy into a physical attack, which he absorbs with his naturally high defense. This activates Weak Armor, raising his speed by 1 stage and lowering his defense by 1 stage. Then he uses Swords Dance, and proceed to wreak havoc. NOBODY sees it coming and nobody knows how to deal with it. He, too, often sweeps entire teams. And it's totally hilarious.
90% accuracy moves eeecckkkk
An enemy Steel type is obviously it's worst enemy. A bulky Electric type will also end this thing like nothin' else (OHAI MAGNEZONE). I don't like the set at all and would never use it, but the surprise value really isn't all the set has goin' for it, so I at least don't disapprove of it. What I do disapprove of however, is that your only Steel type can't really switch into anything. Better keep that Mamoswine alive, if ya catch my drift.
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Salamence: Mixed Sweeper
Attachment 10156
Rash, Life Orb, Intimidate
64 Atk / 192 SpAtk / 252 Spd
Earthquake
Flamethrower
Dragon Claw
Draco Meteor
Typical stuff. He has maintained a spot on this team because he has nice synergy with Mamoswine. I can't really explain why, but hey. They form a nice offensive core that can hit hard without any boosts.
Dragon Claw off of neutral nature and 64 Atk EVs is laughable at best, you want Outrage, no exceptions. Draco Meteor to an Outrage breaks any wall that dares stand in your way after all, thanks to both moves' superior base power. Fire Blast is also really important too IMO, the higher base power is necessary when you're not running Specs/maximum SpA investment.
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Mamoswine: Physical Sweeper
Attachment 10157
Adamant, Leftovers, Thick Fat
252 Atk / 252 Spd / 4 HP
Earthquake
Ice Shard
Icicle Crash
Stone Edge
SUPER COMMON MAMOSWINE? BUT WITH THICK FAT TO SORTA COMPENSATE FOR MY TEAM'S FIRE WEAKNESS? SIGN ME UP
super common mamoswine? a-ok
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Conkeldurr: Most Valuable Player
Attachment 10159
Brave, Leftovers, Guts
120 HP / 252 Atk / 136 SpDef
Bulk Up
Drain Punch
Mach Punch
Payback
This guy. Simply RIPS through teams. He has decent bulk to begin with, but Bulk Up helps get that higher. Sometimes, though, he just uses Drain Punch to regain pretty much all of his health. He can do that because he's a baller. Payback covers psychic types (which, thanks to his specially defensive build, he can do with ease) and ghosts. Mach Punch deals with frail sweepers that could otherwise kill him. He does NOT go down easily.
super common conkeldurr? a-ok
Use this guy myself on occasion, including this very set. >______>
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THE END WHAT DO YOU THINK