2manygentlemen
Chieftain
Based on a combination of my experiences, along with general consensus and this leader guide on GameFaqs, I've made a tier chart of the 52 leaders based on how strong they are in general, and whether they're more well-rounded or specialized. Note that the well-rounded/specialized axis isn't terribly precise, as that was largely guessing on my part and there wasn't a huge amount of space to work with, so it's to be taken with a grain of salt. Also, in the top couple tiers, the well-rounded/specialized axis is relative to other leaders in the same tier. (Obviously the higher tier leaders are all generally better rounded than the lower tier leaders in general)

The tiers go as follows:
S Tier (Top row): Stand head and shoulders above the rest. Have great traits, as well as powerful uniques, some (Egypt, Rome, Incas, Persia) of which can break the game with a successful rush.
A Tier (2nd row): Not quite as strong as S tier, but still very good. They all have great trait combos and/or uniques that may also synergize well, too.
B Tier (3rd and 4th row): These leaders are good, but don't quite have the same potential that S and A tiers do. Some here do have a great base for one particular thing (say warmonging or wonderbuilding), but aren't very well-rounded, while others are but don't excel in any particular area. Some also have particularily poor uniques.
C Tier: These leaders are okay, but are set back somewhat by A. being too specialized towards warfare at the expense of other areas, B. having lacking uniques and/or C. having a trait combos that don't have much synergy
D (for Dice) Tier: These guys are very reliant on either warfare or religion/culture, to the point that their most optimal way to leverage their traits and/or uniques depends heavily on one area. If they're not able to take advantage of this, you're basically working on your own, without any benefits.
F Tier: These guys have very poor traits that don't combine to produce anything interesting at all. Though Charlemagne and Mao at least have interesting unique buildings and units, respectively, those aren't much consolation to make up for lacking trait combos. Sitting Bull doesn't have much to show for in uniques and his protective trait does little to help leverage the Philosophical trait.
With that said, the chart's all just my opinion/speculation. I'd love to hear what you agree and disagree with regarding my chart. Feel free to post your own views and ideas regarding who's the best or worst, perhaps even your own tier lists or charts.

The tiers go as follows:
S Tier (Top row): Stand head and shoulders above the rest. Have great traits, as well as powerful uniques, some (Egypt, Rome, Incas, Persia) of which can break the game with a successful rush.
A Tier (2nd row): Not quite as strong as S tier, but still very good. They all have great trait combos and/or uniques that may also synergize well, too.
B Tier (3rd and 4th row): These leaders are good, but don't quite have the same potential that S and A tiers do. Some here do have a great base for one particular thing (say warmonging or wonderbuilding), but aren't very well-rounded, while others are but don't excel in any particular area. Some also have particularily poor uniques.
C Tier: These leaders are okay, but are set back somewhat by A. being too specialized towards warfare at the expense of other areas, B. having lacking uniques and/or C. having a trait combos that don't have much synergy
D (for Dice) Tier: These guys are very reliant on either warfare or religion/culture, to the point that their most optimal way to leverage their traits and/or uniques depends heavily on one area. If they're not able to take advantage of this, you're basically working on your own, without any benefits.
F Tier: These guys have very poor traits that don't combine to produce anything interesting at all. Though Charlemagne and Mao at least have interesting unique buildings and units, respectively, those aren't much consolation to make up for lacking trait combos. Sitting Bull doesn't have much to show for in uniques and his protective trait does little to help leverage the Philosophical trait.
With that said, the chart's all just my opinion/speculation. I'd love to hear what you agree and disagree with regarding my chart. Feel free to post your own views and ideas regarding who's the best or worst, perhaps even your own tier lists or charts.