The ultimate melee unit for the Zerg swarm without a doubt would be the Ultralisk. It has a large damage per hit statistic and also inflicts cleave damage which means it is very useful against weak, massed units such as the Zergling or Marine. If that was not enough, it has a special attack vs buildings, which enables them to raze a base fairly fast. They do cost a lot, so it would be wise to ensure one has a strong economy before teching to Ultralisks. It is also worth noting that they have 600 hit-points and rather good base armor, which means they are ideal to soak up damage, doing that job even better than the traditional meat shield, the Zergling.
