Counters are generally what you would expect. Percent based damage is great against him, so things like Malthael and Tychus give him a hard time.
A more surprising one in my experience are fast-moving, auto-attack based heroes like Valla and Lunara. Because Deathwing has long wind-ups, when he’s fighting out in the open against these kinds of heroes they run circles around him and have an easy time dodging his stuff while killing him. His best bet in those scenarios is to run away a bit then suddenly turn around and catch them with an Onslaught > Auto Attack combination.
He’s also a little weak to poke, things like Hanzo and Chromie can whittle him down.
He’s great against teams with lots of Melee, since his abilities all pierce and generally do damage in wide areas. He’s also uniquely good against heroes like Garrosh and ETC since he doesn’t have to respect their normal areas of threat that they control.
More than excelling against certain heroes I feel he excels on certain maps. Maps with lots of choke points like Infernal Shrines are great for him because of how much area he can deny with his various abilities.