For C3C, I find the best traits to be Agricultural, Scientific, and on Arichpelago maps, Seafaring. Religious is good too, industrious is ok, imo (I tend to create an early worker pump).
I fin Commercial, Militaristic and Expansionist to be poor picks, especially commercial which I see as a waste. Expansionist can be goo on Pangea maps, as it can allow you to find the AIs before they find each other, and get tech in goody huts, such that you can be tech broker and keep up with the AI. Seafaring in archipelago is good for the same reason. Both are decent in continents, but 2nd tier.
My choices for best civ are:
Sumerians (two great raits + UU is very helpful defensively: half price spearmen at game start. Great as a builder civ).
Celts (Agricultural and Religious is very good, and the 3/2/2 swordsmen is a great UU).
Dutch (Agricultural + Seafaring is excellent for Archipelago, UU is basically a cheaper musketman that you get earlier, so also good for defense).
Byzantines (Scientific + Seafaring is great for Archipelago, UU is also good on archipelago).
Iriquois (For Pangea, expansionist is good, and their UU is excellent, a 3/1/2 horseman, similar to the Celtic UU).
Persians (Excellent offensive UU, and Scientific + Industrious is good).
I tend to play on Deity level now, some Demigod.
My priorities in picking a civ are:
1) Ability to find AIs fast, so as to be able to broker tech, and not to fall an Age behind in tech. This means Seafaring (archipelago map), Expansionist (Pangea map), or at least starting with Alphabet to make Curraghs. If a civ doesnt have this ability, I'll have to find a way to cope, generally by attacking a lot in ancient and getting techs for peace, or trading for Alphabet ASAP and getting curraghs looking for far off civs.
2) Strong growth potential early (Agricultural gives the biggest boost to this, with food bonus and immediate granary tech to set u pa settler pump fast). Thsi is to keep up with or near the AIs in growth
3) Good UU. My UU should give some significant offensive or defensive strength, either in the ancient age, or early middle age.
Finally, the civ should either be a strong builder civ (Scientific and Religious help this, especially scientific for cheaper libraries and universities, and more SGLs) or strong attacking civ (great offensive UU like Gallic Swordsmen or Mounted Warrior).
So theres about a half dozen civs I think are top tier, and which is best depends on map type, and wether I want to focus more on building or attacking.
If I could pick any combination of things, for all map settings, I would probably want Agricultural and Scientific with a very strong attacking UU such as Immortal, Gallic Swardsmen or Mounted Warrior. But that doesnt exist.
