Hmmmm... So you want to be the best, eh? OK, so both work out great and IMO both are equals, but in the past Ferrothorn has been one of the most reliable I have used. Don't worry both have good and bad qualities but that were going to get to later.
Ferrothorn @ Leftovers/Rocky Helmet
Trait: Iron Barbs
Nature: Careful/Impish
EVs: 252 HP/128 Def/132 SDef
Leech Seed/Curse
Gyro Ball/Payback
Thunder Wave/Stealth Rocks/Spikes
Power Whip/Seed Bomb
This is the standard Ferrothorn. Lefties is to get back the HP you barely lost. Rocky Helmet with do loads of damage with Iron Barbs. If Cloyster starts a sweep, send in Ferrothorn, which by getting hit does more damage to it than you'll actually take. Careful is better than Impish except against Conkeldurr. Curse is to maximize damage for Gyro Ball and Payback.Gyro gives STAB while Payback rids of others... Use Thunder Wave with Leech Seed, though Hazards are way better in general. Power Whip for damage, but I'd be a millionaire if I had a nickel for everytime it missed, so Seed Bomb is there if you have trust issues.
Forretress @ Leftovers
Trait: Sturdy
Nature: Impish
EVs: Same as Ferrothorn's
Toxic Spikes
Spikes
Stealth Rock
Rapid Spinner
Though not as good as Ferrothorn, but it's a better lead. Set up Hazards Spin'em off if they make haste doing so. That's basically it. If you want a lead Spiker, try Skarmary.
Both are good, both weak to fire, both got good bulk. But in all seriousness, Ferrothorn definitely.