Which new civs do you think will be the hardest to play against ?

Brasil is destined to become a powerhouse.

It's hard to say without playing, but I'd guess Shoshone or Poland.

Shoshone depend a lot on how the AI can use Pathfinders, and the limits of AI happiness. If the AI can still rapidly expand early, then they can claim even more land than usual, really starving nearby players for land. But the real danger is the Pathfinder. The AI should start with 3 Pathfinders, and if they successfully use them to gobble up ruins (and choose the proper rewards) then the AI becomes even more impossibly good at the crucial early stages of the game.

Poland will just be a ***** to war with. The Winged Hussar can seriously F up your battle formation, exposing the weaker units (CB and XB, Artillery) to danger. Additionally, Ducal Stable gives them early gold that seems elusive in BNW, and additional social policies are nothing to scoff at, especially since they receive the social policy as new trees become available. This is more dangerous in a good player's hands (no more timing social policies to click as you hit the Renaissance!) but 7 additional social policies throughout the game will surely help the AI, as well.

But hey, since the game isn't out yet, I have no idea! Before playing, I'd have never guessed that Hiawatha would be my Most Feared Enemy!

Now that I've seen the flavor values I'm changing my mind and picking Zulu.

Shaka has the highest expansion motive in the game. The current highest (Hiawatha) is never not a runaway in games where he's given even a modicum of space to expand into.

:0

But will the AI be able to expand as easily in BNW, since their happiness bonuses are nerfed?

I only voted for two, but most of them are going to be tough in some way. Most threatening, IMO, is Zulu and Morocco. Zulu for obvious reasons - Impis just seem terrifying to go against - and Morocco because between Berbers and Kasbahs, they're gonna be hell to invade. And that's just militarily, they're also gonna be rich.

Assyria seems like they'll be a threat on any difficulty that you're outpacing them do science, and the siege towers are annoying.

Shoshone is just going to be annoying. They're gonna have pathfinders galore, they'll be grabbing piles of land when they inevitably spread everywhere, and the bonus in their own territory is gonna make them harder to take out.

Poland is gonna have some extra gold and policies, but they'll mostly just be a threat when they've got their Winged Hussars. Venice is probably gonna be terrible in the AI's hands, but they're gonna be jacking CS which will suck.

That leaves Brazil and Portugal. Brazil's bonuses kick in way too late for the AI to use effectively, and Portugal's may also be outside the AI's comprehension, but then again we don't know how that AI uses their trade routes, so.

I voted Assyria and Morocco. Assyria because war and science is a deadly combination in Civ5. Morocco because I think their trade routes and Casbahs will be a very strong combination.

1st place:Zulu

9th place: Indonesia

Zulu might end up with the same problem the Hittites have: They anger so many of their neighbors that they get wiped out/stunted early.