Unfortunately, I'm not aware of any hard data of the sort that RftG or Dominion players collect.
That said, Bilkis (once per turn, pay 1 to the bank for 1 of any resource) is my pick for best all-around leader. She lets you get away with playing fewer resource cards and avoids enriching your neighbors.
Other choices are more situational:
Imhotep (resource discount for Wonder stages) is great for Giza B or Wonders that have an inconvenient cost.
Hammurabi (resource discount for blue cards) is good unless you're committed to a maximum-science strategy.
The vanilla VP leaders are pretty good for their cost, and since in general you will be happy to pay 1 coin for 1 VP, the largest ones, Cleopatra and Nefertiti, are strongest.
If you can get two leaders that support either science or military, that can be strong. It does sort of commit you to that path, so if you draft 2 military leaders and your neighbor decides to pursue a military strategy, you might find yourself frustrated. Either one, although science moreso, is high-risk/high-reward.
Of the military leaders, Caesar and Hannibal grant shields, which give you a leg up and signal to your neighbors that you're not worth fighting. Alexander and Nero give you bonuses for winning. Pericles is expensive, but can be worth 8 or 10 points if you're going military. Leonidas provides a discount for military, although if you often get a fair number of brown buildings anyway you might not need the discount.
Of the science leaders, Archimedes provides the discount, which (in contrast to military) can be important especially in Age 1 since the science costs require advanced resources. Hypatia and Aristotle make your green cards and sets worth more. Euclid, Ptolemy, and Pythagoras provide science symbols; they're expensive but can be worth lots of points if they fill up a set or give you the 5th card of a kind.
