Well, I think the best trait depends on your style of play, ie. Religious is good if you change governments a lot, which I don't, so I don't use the trait. Cheaper temples too, but I've always prefered to build libraries.
Scientific, is very useful for high culture, cos of cheap libraries, but I'm always able to use a few trades, followed by about 60% research to be more advanced than the AI, so I don't really use it.
Militaristic- I've always used my leaders for wonder-building, and once I get Sun Tzus, my army's fine for the rest of the game, really.
Agricultural- I could see how this would be useful, especially since food, unlike everything else, never has corruption, but I can expand easily enough in the Ancient Age, and after that, well military is what matters, not settling. Plus, high population= Civil Disorder, since I don't like building temples.
Seafaring- well, the extra commerce is absolutely USELESS, but my playing style will always have a place for a quick navy (I LOVE navies- even though in practice they're actually fairly useless outside of Ancient and Modern ages), plus cheap harbours are very useful for far-flung islands with serious corruption but good resources.
Expansionist- I always prefer to use my navy to explore, then trade world maps later on, and I never (EVER) build scouts, or have problems with expansion so I never use this trait, especially since I play on 'Pelago and Continental maps, never Pangea.
Industrious- I LOVE THIS TRAIT!

The shield bonus isn't much, but the real power is with the workers- mines are 2 TURNS quicker- and that's a lot. That means 50% more mines (well, OK, by the ADs, most people have all their core production mined, but still) , which means, for a while at least, 50% more production (kind of)! This also, of course means you only need to build less workers, which means more population too.

All of which add up to make this by far and away my favourite trait.
Since Seafaring and Industrious suit my style best, I (nearly) always play as Carthage.
Oh yeah, while I'm here, does the "build preferences" of each nation mean anything? Carthage's is Naval units and Trade, I think, but I haven't seen any cost differences...