The Secret World, hands-down.
Nothing about The Secret World comes easy. The combat has you struggling sometimes against standard mobs, the game's many systems are complex and non-intuitive, the story isn't spelled out for you, and some of the quests tax my brain far more than my reflexes. Don't get me wrong; I love TSW to its core, but it's stupid hard at times, so much so that it's kept me from seriously pursuing an alt.
What would you say is the most challenging MMO that you've ever played?
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