Ferrothorn -AKA death to the Bulky Water type. Ferrothorn is the near universal water type counter and the main reason why many fell out in Gen V (besides power creep). Even then that is not all Ferrothorn is toting. Its Steel typing is also a valuable asset against Fairy types, of which only a few possess reliable tools to take it out. Excellent defenses, a great movepool, anyone building a team in singles must watch out for Ferrothorn.
Pros:
+Grass/Steel typing -9 Resistances +1 Immunity is a fantastic exchange for two weaknesses (albeit one being a 4x weakness).
+Defenses - 74/131/116 Defenses are fantastic and can take a great deal of abuse
+Movepool - Offense, boosting, hazard setting, Ferrothorn has almost everything it could ask for
+Ability -While Iron Barbs does not directly boost Ferrothorn's bulk, physical contact causing 1/8 of the attackers HP is significant
Cons:
-4x weakness -Fire is a major achilles heel for Ferrothorn and it will be maimed if not outright KOd by most Fire type coverage.
-Slow -While this is a boon in Trick Room, out of it Ferrothorn will be taking a hit before doing anything. Only Stakataka is slower in common competitive play.
-No Direct Recovery -Ferrothorn lacks any potent healing moves like Synthesis, Shore Up or recover, forcing it to rely on Leech Seed and Leftovers or Rest for recovery.
Metal Durian
Ferrothorn
Ability: Iron Barbs
Item: Leftovers/Rocky Helmet
-Leech Seed
-Spikes/Stealth Rock
-Gyro Ball/Power Whip
-Thunder Wave/Toxic/Protect
Nature: Impish (+DEF, -SATK) or Careful (+SDEF, -SATK)
EVs: 252 HP/252 SDEF/4 DEF
Standard Ferrothorn set. Leech Seed provides damage over time and recovery, but is walled by opposing Grass types. After that, you have your choice of Entry Hazards, -Spikes can regularly cause more damage but takes longer to set up, while Stealth Rock damages just about anything and is quicker to setup. Next is STAB move of choice. Gyro Ball can hit fairies, but has poorer PP and can be unreliable, while Power Whip has poorer accuracy, but better coverage and more reliable base power. Last move is up to preference. Thunder Wave and Toxic spread around your status of choice, (Toxic adding up quickly with Leech Seed, entry hazards, Iron Barbs, etc.) while Protect allows you to stall out for Leech Seed recovery and scout for moves.
EVs and nature maximize bulk. Leftovers makes you more bulky, while Rocky Helmet combines with Iron Barbs will lose 14% (?) of their max health.
Other Options
Curse -Ferrothorn makes a passable Curse Sweeper. +ATK and DEF, and the cost of the stat Ferrothorn is already horrible at.
Knock Off -Everything loves Knock Off!