Don't use this:
Forretress @ Leftovers
Ability: Sturdy
EVs: 252 HP / 4 Def / 252 SpD
nature: Relaxed
- Stealth Rock / Spikes / Toxic Spikes
- Rapid Spin
- Gyro Ball / Volt Switch
- Volt Switch / Spikes / Toxic Spikes
Why it's bad:
Forretress isn't necessarily "bad" per se, its just simply outclassed by so so much. If you need a pivot that can remove hazards, then you can use Scizor. But if you go for the most common utility set, then you can instead use Ferrothorn. Forretress, yes, it does have the advantage of having rapid spin, but it doesnt have any recovery. Forretress also has absolutely miserable special defense, so it can only function as a defensive wall or maybe a mixed if you give it max spdef and a boosting nature, but all that is wasted on a stat that only has 60 bas special defense.
Instead, use this:
Ferrothorn @ Leftovers
Ability: Iron Barbs
EVs: 252 HP / 216 Def / 40 SpD
Nature: Relaxed / Impish
IVs: 0 Spe
- Leech Seed
- Protect / Thunder Wave
- Power Whip / Gyro Ball
- Stealth Rock / Gyro Ball / Spikes
Why it's better:
Ferrothorn is an absolute beast. Having absolutely insane 74 / 131 / 116 defenses with an amazing ability, decent typing, AND this great moveset makes Ferrothorn an absolute necessity to any especially bulky or stall team. Gyro Ball also has a surprising amount of power as well due to its laughable 20 base speed. Not to mention, its 94 base attack is pretty good actually, especially when keeping all its other stats distributed as well. Also, who doesn't love a great hazard setter with leech seed. Once ferro has leech seed up, your pokemon is paralyzed, chances are that this guy is going to have all its hazards up before you even know how to react.
Conclusion:
Ferrothorn is almost better in every regard except for ferrotresses moveset when it comes to Volt Switch and Rapid Spin. However, all of Ferro's other attributes greatly outclass the poor man's ferrothorn. If that doesnt prove it to you, check out these calcs:
252+ Atk Garchomp Earthquake vs. 252 HP / 0 Def Forretress: 135-159 (38.1 - 44.9%) -- guaranteed 3HKO after Leftovers recovery
252+ Atk Garchomp Earthquake vs. 252 HP / 252+ Def Ferrothorn: 108-127 (30.6 - 36%) -- guaranteed 4HKO after Leftovers recovery
0 Atk Ferrothorn Power Whip vs. 252 HP / 0 Def Azumarill: 296-350 (73.2 - 86.6%) -- guaranteed 2HKO
0 Atk Forretress Gyro Ball (32 BP) vs. 252 HP / 0 Def Azumarill: 39-46 (9.6 - 11.3%) -- possible 9HKO