I know it's getting more and more popular, but I'd have to plump for Gastrodon here. It completely shuts down Rotom-W and (with HP investment) comfortably Recovers off CB U-Turn or Superpower from Scizor. I've used it in one of two ways in the past and loved both:
Gastrodon (F) @ Leftovers Trait: Storm Drain
EVs: 252 HP / 4 Def / 252 SDef
Calm Nature (+SDef, -Atk)
- Substitute
- Recover
- Toxic
- Earth Power
Comes in on a choiced Toed or Rotom-W and gets up a sub, toxic stalls pretty admirably along with having decent power on its STAB with +1, even with no SAtk investment. Has some problems due to mono-attacking coverage of course, so I suppose you could go Scald > Substitute, but that's preference.
On the other hand, I've also worked with this set.
Gastrodon (F) @ Life Orb Trait: Storm Drain
EVs: 252 HP / 252 SAtk / 4 SDef
Modest Nature (+SAtk, -Atk)
- Scald
- Earth Power
- Ice Beam
- Recover / Hidden Power (Fire)
Still an amazing Rotom-W counter. With a Storm Drain boost this thing gets to 465 SAtk without factoring Life Orb, which is pretty hot. At +1 HP fire outside of rain can OHKO even 252/252+ SDef Ferro 81.25% of the time, while with one layer of spikes this becomes a guaranteed OHKO. (standard Ferro being one of the only things Gastrodon can outspeed so no Power Whip worries), whilst its STABs crush most things that are neutral or better. Has the additional bonus of not being hurt so bad by getting Tricked a choice item (it particularly doesn't mind specs) as so many Trick mons would do predicting the more used Toxic stall set.
All in all I've found Gastrodon to be pretty versatile and very anti-meta with Volturn being so rampant. Just watch out for surprise HP Grasses.
Another mon I've been playing around with recently is Aggron. With DDNite, CBNite, CB Haxorus etc being rampant, I'm finding my teams getting ripped apart by Outrage pretty commonly, and being so hipster I've been trying to find something other than Skarm that can switch in on Outrage and wreck. So after some playing around I got to the following Aggron set:
Aggron (F) @ Leftovers
Trait: Rock Head
EVs: 248 HP / 252 Atk / 8 SDef
Adamant Nature (+Atk, -SAtk)
- Thunder Wave
- Head Smash
- Hone Claws
- Substitute
With 180 base Defense and a Dragon resist Aggron laughs off any Outrage and either sets up a sub or paralyzes there and then, and after that with any luck I can sub until they get fully paralyzed and set up a Hone Claws, after with point Head Smash is just insanely powerful on anything that doesn't resist it or isn't Gliscor. Of course the main issue with Aggron in OU is that it's quad weak to two common attacking types, but with team support these problems can be rectified (for instance I support it with Intimidate Gyara, who eats up water attacks, -1 Close Combats, and is immune to EQ.)
Obviously Aggron has flaws in OU, and I'm certainly not saying he can just be dropped onto any team. I use him on my current set because he fills a purpose nicely, but I've been pleasantly surprised by just how many kills he can get per game. (ie pretty much always one, and with some parahax, more.)
Other cool mons are +Def Claydol (nice Terrakion counter, hot spinner due to Levitate and SR resistance and can also put up rocks itself, so imo a better Donphan), Rotom-C for reasons covered in prior posts by better players, Staraptor which has been discussed to death, Victreebel in sun and Roserade. I love Roserade with a more offensive set than the sleeper/spiker since it also gets Weather Ball so has nice utility on both Sun and Rain (although fu Ferrothorn) teams.