(I'm going to post this before Thund steals it)
So now that Umbreon is gone, I felt inclined to experiment a bit with Missy. I found a set that works really really well against Stall. It shuts down most stall members and forces a ton of switches.
Mismagius @ Leftovers
Timid, 176 SPE, 236 HP, 96 SDEF
-Taunt
-Substitute
-Will o Wisp
-Pain Split/Rest
On paper, this might seem like an inferior bulky missy (it trades off Shadow Ball for Substitute and sticks the SATK EVs into HP) but while it loses usability against offensive teams it gains much better stallbreaking ability. The main problems with bulky missy are that, even with Will o Wisp, it takes a ton of damage from physical attacks, especially those from the likes of Absol or Rhyperior, it is vunerable to being revenge killed (especially by Swellow) or pursuited, and it is vunerable to side effects of attacks (sludge bomb poison, ice beam freeze, etc). This set aims to fix those issues with Substitute. Burn your opponent, then substitute down as they try to attack you. Your substitute will be unbreakable against weaker stall members (Registeel can't break your sub with Iron Head, for instance) and, against those who can break it, you're free to sub down until you need to heal. You can use Pain Split or Rest, depending on your preference. Rest works best with a cleric, but it does fine by itself too, especially if your opponent's physical attackers are all burned. Pain Split lets you heal without wasting turns, although if your opponent leaves his weakened wall in to die you're not going to be able to heal very easily.
This will cost stall teams at least one Pokemon if they don't have a Pokemon that is immune to Will O Wisp and has reliable healing (Clefable, Arcanine, Moltres). A standard stall team consisting of Milotic/Venusaur/Hitmontop/Chansey/Spiritomb/Rhyperior, for instance, will simply not be able to beat this set and will lose its members one by one. Speaking of Hitmontop, this is probably the best spinblocker ever. It can Taunt Foresight and Rest attempts, burn the spinner, and heal off any damage.
This pairs extremely well with Pokemon that only have one real counter on stall teams, because your opponent will often sac a Pokemon just to bring Missy down to low health. For instance, I pair this Missy with Arcanine, Rhyperior, and Jynx. If they sac Milotic/Venusaur/Registeel, one of my team members will dismantle the rest of the core.
This set has a couple of issues. One is Swellow. If you sub on the switch though, you can beat it by Subbing until you're at <25%, then using Pain Split/Rest. Swellow should be dead or close to it by this point. Another one is Clefable. Taunting Clefable makes it completely useless though, so you can happily switch in a boosting attacker without worrying about Encore/Twave. And finally, bulky fire types give this major grief. You can Taunt Arcanine/Moltres and prevent them from healing; you may even beat Moltres, but Arcanine does too much damage with Flare Blitz. Stealth Rock and Bulky water support is recommended; you can also switch to Toxic if you're more worried about those guys than Registeel/Steelix/Venusaur, which lets you beat them.
Give it a shot if stall is bothering you.