Iron Fist for the sake of Meteor Mash. It's a waste that the coding lists the move under the ability yet no Pokemon with Iron Fist can learn Meteor Mash.
The other abilities don't make as much sense on it, Arena Trap especially. The Japanese name for Arena Trap refers to the antlion (the insect that the Trapinch line is based on) and the way they dig bowl-shaped pits in the sand. It's the reason why only Trapinch and the Diglett line have Arena Trap.
Iron fist. Its fists are made of iron, after all xD It has so many punches to make good use for it... I don't know if that ability would make Metagross overpowered, but surely would be a great alternative to Clear Body.