Abathur. Extremely unique and very difficult to understand from an outside perspective, able to support allies anywhere with minimal warning and safely collect experience across multiple lanes. Also capable of several severe violations of the game's standard design expectations, which is very powerful if leveraged well, but is especially strong against players who don't even realize these things are possible.
The Lost Vikings. A very demanding 3-Hero team whose intended specialty is soaking experience for the team, leaving their teammates free to focus on Objectives, ganks, and similar. The current state of the game has reduced the value of this without them receiving an overhaul, unfortunately, but they're still quite viable! They're also one of the most demanding Heroes of the game to play to their full potential, or honestly even to just a modest level of usefulness.
Medivh. Possibly the most skill-intensive Hero in the game, Medivh is a Support who creates unique opportunities to his teammates by letting them teleport, turns enemy burst damage into healing for his team if he has good timing, and can roam the battlefield with impunity in Raven form, monitoring enemy actions and hunting for vulnerable foes with his enemies completely unable to stop him.
Zarya. A durable Support who Shields allies and translates successfully protecting her allies into more damage output from herself.