Heart of the Swarm, despite being billed as an expansion, is practically a new game unto itself. Indeed, it's even been described loosely as the second part of a three-part trilogy beginning with Starcraft 2: Wings of Liberty (the original game) and ending with Starcraft 2: Legacy of the Void.
In addition to adding new units and tweaking the game here and there, Heart of the Swarm added 20 new campaign missions to the mix. The new missions focus on fan favorite Sarah Kerrigan and the Zerg race, providing a much different perspective on events.
Most importantly, it's on this list because it's the only release so far this year that gave Starcraft junkies a good excuse to play more Starcraft 2.