He’s got an outstanding defense and HP and you have only attacking moves, so this one is an obvious choice. It gives him amazing bulk, which complements his massive attack stat.
A common item for bulky attackers, which basically defines Melmetal. The gradual healing is helpful as he has no form of recovery outside of it.
This one takes a more offensive approach. The 50% boost to his already massive attack makes Iron Fist and STAB boosted Double Iron Bashes do MASSIVE damage.
So it depends on how you want your Melmetal. If you want him to be more bulky, go assault vest. if you want him to hit hard, go choice band.