Overall, probably Portugal.
Venice is great, and the extra trade routes are very, very worthwhile, but you have to be able to defend yourself, which is tough with Venice, and in the games I've played the AI has known enough to keep a target on Dandy's back. Glass cannon, financially-speaking, but very powerful in the right hands.
Morocco is strong, but more situational than you'd think. Start too isolated and you're screwed (something that doesn't really happen with Portugal, due to the more naval focus of their trading.) Like Arabia, Morocco benefits from a gung-ho piety start. Desert Folklore is a no-brainer here. Murder if you have to in order to get Petra, even if it's not as good as it used to be.
Arabia: basically everything I said about Morocco. They play very similarly, at least to me, so far.
Netherlands: Got screwed in the expansion. Not as badly as feared, maybe, but still. They were built for G+K and not at all redesigned for an entirely new game here. I'm eager, very eager, for someone to prove me wrong and find an awesome new exploit for them, but it hasn't happened yet.
Carthage: Effectively Arabia of the Sea, in terms of the most important civ bonus. Arabia gets the extra religion spreading, Carthage gets mountain passage, a little more gold, and MUCH BETTER TRADE BONUSES. Until the medieval age, anyway, but by then you should have your snowball rolling anyway. Give it another week or so and people will come back around to Carthage in a big way.
Portugal. Yeah, the Naus and Feitoria don't arrive until later. When they do, though, they blow up like crazy, especially the Feitoria, and you've already gotten your more-boring-but-more-important trade toue bonus from turn one. If you want to play financial, and not take the gamble on Venice, Portugal's your civ. Followed closely by Carthage, Morocco and Arabia.