Multiplayer is a core characteristic of practically all big games nowadays, for better or worse. Sure, there have been some very successful titles that didn't feature any sort of online mode, like Dishonored or The Walking Dead, for example, but they were largely exceptions that reinforced the rule.
While many titles had impressive online experiences this year, ranging from pure competitive ones to cooperative-focused modes, one game stood out through the depth of its multiplayer experience and through the constant changes made to it: Mass Effect 3.
At the beginning of the year, BioWare revealed to the world that the long-awaited final installment in its role-playing shooter series would feature a cooperative multiplayer mode.
This caused a backlash in the community and many thought it would just be a tacked on system that had nothing to do with the prestigious franchise made by the studio.
Fortunately, that wasn't the case, as the cooperative mode in Mass Effect 3 delivered an intense and equally complex experience, even if its actual mechanics were relatively basic: survive 10 waves against increasing enemies as well as a final extraction one.
Just like in the single-player Mass Effect experience, you had different classes (Adept, Soldier, Engineer, Sentinel, Infiltrator, or Vanguard) but also different races (Human, Asari, Turian, Quarian, or Krogan), each with different powers that could be evolved in various ways and equipped with all sorts of weapons.
While the base experience was pretty addictive, BioWare didn't settle just with a moderately successful multiplayer mode, and began releasing all sorts of new content that was given away for free to all players. From new weapons to new alien races (Vorcha, Batarian, or even the tiny Volus), as well as the all-new Challenge System that adds another incentive to keep playing well after the game was initially released.
For its polished mechanics and for BioWare's constant updating and the free DLC packs, Mass Effect 3's multiplayer mode is the best of 2012.