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Depends what you mean by action.
At the moment Ebonheart is the most populated followed by aldmeri and then Daggerfall which obviously effects PvP.
In terms of the campaign they all have an equal number of enemies, dungeons, quests and places. However I personal enjoy Daggerfall the best for its settings and story with aldmeri being a close second.
If you don't have the explorer pack you should focus more on the race of your character than the alliance as you will eventually get the play them all but you only get to pick your race once.