Well, first off, which ability does it have? That's what makes all the difference. Personally, my own has Rock Head, so neither Take Down nor Double-Edge will do recoil damage. Sturdy... well, it's good, but really, all of the OHKO moves are inaccurate, and unlike in Gen II, we can't have a Mind Reader-Fissure Poliwrath. Iron Tail and/or Metal Claw are also good, since both have that chance to raise Attack or lower the opponent's Defense, which in turn causes the attack itself to be more powerful. Other than that, Earthquake might be a good choice, it's pretty much always a good choice. I'd definitely not get it Surf, though, since it has pretty low Special Attack, and a Steel-type or Ground-type move weakens pretty much everything Water does, anyway, along with a few other things.

...and if your Aggron doesn't have Rock Head... um... Breed another one. Preferrably with Adamant nature, if you're going for Attack moves over Special Attack moves. Maybe get it Body Slam, too, that's an Egg Move. You can breed that from a Meganium, if you have one. And if not, Tropius works.

And just so you know, Gentle nature decreases Aggron's already tank-like Defense in favor of increasing its Special Defense.