There is one move I'm completely baffled as to why Metagross doesn't get: Heavy Slam.
It would be Steel STAB, coming from the second heaviest Pokémon in OU. And it would fit Metagross perfectly. While most of its Pokédex entries comments on its awesome brain(s), the one that doesn't says this:
"
Metagross is the result of two Metang achieving fusion. When hunting, this Pokémon pins the prey to the ground under its massive body. It then eats the helpless victim using the large mouth on its stomach."
Picture Metagross attacking its helpless victim (say, a parked car. Steel Pokémon eat metal after all). How does it do it? Of course, it could seize it by a mighty claw, lift it high in the air, and slam it full-force into the ground, mashing it between the ground and a hammer-like leg (Meteor Mash). Or, if the prey is larger than a Vespa scooter, it would silently hover above it, bravely defying all of gravity, then let itself fall, hitting the target with all four legs. Then it opens it mouth and devours the scraps. After consuming its meal, it hovers away, leaving an unsuspecting commuter one hell of a surprise the morning after.
So yeah, Heavy Slam for Metagross, please.