Ha! It's obviously Magnezone.
Much as I regret to say it, Magnezone's role in OU is limited to the niche of steel trapping and subsequent pivoting, and this niche is slowly but surely shrinking with each new generation. Magnezone can't make adequate use of its bulk and 10 resistances due to its extremely exploitable weaknesses (especially to ground; I mean why the hell does it not have levitate) and the fact that it is rather underwhelming in power if it forced to use a Choice Scarf.
Mega Magnezone should obtain a dramatic increase in Special Attack and Speed, along with minor bulk buffs, as well as the ability Levitate. (It's not uncommon for a fat metal monster to become fast when it gets its butt off the ground. Look at metagross's speed boost, and it's mega doesn't even have levitate)
That way, Magnezone can trap steels, eliminate them and proceed to mega evolve on that turn, giving it a relatively safe and free turn to increase its speed and power, and take on the role of a fast, powerful special wallbreaker and pivot while still keeping Magnet Pull in its unevolved state to perform it's original job.