I'd say leech seed. Based on my competitive experience I'd say leech seed since gyro ball is THE best offensive move ferrothorn can have in this metagame, surpassing power whip for reasons I won't go into unless you ask me.
But moving on, it's good to have a Pokemon like ferrothorn use its decent attack to well...attack, but ferrothorn is primarily a wall and in your case it's one that sets up, and since you plan on using a ferrothorn with two hazards(which is ok), you don't really have the moves to actually ferrotroll(ha pun) such as twave or protect. Especially without protect, you don't really have a way to recover, and since ferrothorn has lots of purposes outside setting up(attacking not really being one of them), you last move can be leech seed, giving ferrothorn a role as a staller, while allowing it to heal itself.
Power Whip wouldn't be a bad idea, but overall leech seed fits into ferro's set better than Power Whip IMO
Ability: Iron Barbs
EVs: 252 HP 216 def 40 spdef
That's the set I'd give you based on what you asked for, but in my opinion, don't limit yourself to that set just because it is full of egg moves and hard to get moves in general. I think using moves like protect and Thunder Wave in the middle and changin it up would be to your benefit. Here's a set I like that I got from smogon
move 1: Leech Seed
move 2: Protect / Thunder Wave
move 3: Power Whip / Gyro Ball
move 4: Stealth Rock / Gyro Ball / Spikes
ability: Iron Barbs
item: Leftovers / Rocky Helmet
evs: 252 HP / 216 Def / 40 SpD
nature: Relaxed / Impish
ivs: 0 Spe
Go ahead(If you wanna listen to my advice) and use that set/sets if you want to make the best use out of ferrothorn