Yes, of course it's been asked before, but...

[Norfleet]

Hmm. I'd probably say that the mandatory turmoil themes, particularly in quantity, are likely the hardest: Tien Chi BK, Tien Chi S&A.

The other noteworthy hard theme is probably Carrion Woods Pangaea, a much maligned theme noted for both its reliance on capitol-only living troops....it's killer dominion....and its completely useless growth scale in light of the above.