What tag should be used for Massively Multiplayer Online Games?
Options that come to mind:
Most common? Combined with rpg for games that are both.
Also common. In contrast to tagging both mmo and rpg.
More verbose suggestion. It has been pointed out that although recognisable and in the spirit of minimising abbreviations, it is technically incorrect - as one could have a massively-multiplayer game that was not online.
More correct. [Removed as the g is basically implied.]