Gul'dan is a Hero I've quickly fallen in love with and has gone right to the top of my choice of first picks. Although his kit shares similarities with others in the Nexus (Life Drain is similar to Leoric's Drain Hope, Corruption similar to Dread Orb and Fel Flame similar to Medivh's Arcane Rift) his kit combined is surprisingly powerful. His Trait, Life Tap, provides incredible sustain when combined with Life Drain and allows him to not only survive for long periods of time, but reduces almost all need for him to return back to base. What's also fantastic about Gul'dan is that not only does he have sustain that's invaluable on objective based Battlegrounds, but his Ultimate ability - Horrify - is incredibly strong. The fear is not only disruptive, but ensures enemy Heroes are displaced enough that some are always easy pickings.
Finally and due to the nature of team fights in Heroes of the Storm, Corruption is more than capable of applying a high amount of damage over time pressure, especially when paired with Fel Flame. Just be careful against Heroes that can slow, root or stun - Gul'dan has no means of escape.
Release a wave of flame, dealing 200 damage to enemies.
Drain the life from an enemy over 3 seconds, dealing 132 damage per second and healing Gul'dan for 188 Health per second.
Call forth three bursts of shadow energy, dealing 204 damage over 6 seconds. Corruption can stack up to 3 times on an enemy.
After 0.5 seconds, deal 120 damage to enemy Heroes in an area and Fear them for 2 seconds. While Feared, Heroes are Silenced and are forced to run away from Horrify's center.
RAIN OF DESTRUCTION
Channel to summon a rain of meteors in an area over 7 seconds. Each meteor deals 165 damage in a small area.