Worst year for gameplay: 2015. I may be biased as a Zerg player, but the last year of HotS was awful from a gameplay standpoint. With the Swarm Host completely gutted, Zerg had very little answer for Terran mech play, and Protoss late game was as strong as ever. Zerg's best option was to try and survive until they can get a massive bank, try to trade efficiently, and remax before dying. Whenever a race is forced into such drastic strategies, it's hard to enjoy the game, and this is around when I took an extended break from the game.
Best year for esports: 2012. The KeSPA switch, offline tournaments nearly every weekend, over a dozen Korean teams with two teamleagues, the beginning of WCS giving the scene some structure, and several tournament organizers that no longer exist such as MLG, NASL, and IPL, 2012 was the apex of StarCraft 2's esports scene.
Worst year for esports: 2016. Matchfixing casting a black eye on the scene and one of the game's best players being banned, KeSPA dropping the game almost entirely and the subsequent end of ProLeague, 2016 was by far the darkest for esports