If it matters, I play Emperor difficulty, and never have played higher than that. Moreover, I play 'epic' length with a few mods that have clear impacts: 8 Ages of Pace which significantly extends the length of the eras (and which I've modified further to tweak the length of each era); the abundant resource mod - I always play with 'plentiful' resources; and a mod that both gives every player an extra scout, extra builder and makes sure that you have the same number and type of starting units as the other players. That last one means I begin with 2 builders, 2 settlers, 3 warriors and a scout (and so does the computer). I also play with
no goody huts.
I have R&F as well as all the DLC. My top five are:
1. Poundmaker - Cree
I have found the game to be both more enjoyable - and somewhat more easy - playing as Poundmaker compared to any other leader. I believe it has to do with (a) the mekewap, as I love unique improvements more than any other type of unique civilization ability, and (b) the trade routes bonus, which ended up being much more lucrative throughout the middle part of the game than expected.
The Cree have it all for me. The Mekewap is probably the most powerful unique improvement in the game, unless going for a culture victory. The trade route expansion of cities has proven more useful than I anticipated. This is due in part to incoming foreign routes also adding to your territory (unless I'm sorely mistaken). I've found it easy to 'cut off' peninsulas or other chunks of land quite easily just by sending a couple different, strategically aimed trade routes early on - something made easy by the free route and trader at pottery. I've also found that my cities' owned tiles expand much more rapidly than when playing as most other civs, which I hadn't expected to be so noticeable when I read the ability. Beyond that, the ability encourages many early commercial districts and founding more distant cities while rapidly expanding - it's important to remember to keep yourself well armed as you'll be spread thin. Fortunately, the Cree tend to stack up trade routes fast and that means a decent amount of gold through the early eras replaced by tons of gold by the time the medieval period has arrived so you can often either leave an army un-upgraded and suddenly upgrade them all in a couple turns or you can buy yourself an army in a couple turns.The ability to gain an allies map upon first becoming allies is also surprisingly powerful now - especially if used strategically. Gaining a map by becoming allies with distant players and neighbors will likely give you a bunch of era points unless you've really done your exploring. You may end up meeting a new player or two, gaining 2 era points for each - same with natural wonders. You may meet new city states which might include that one you happen to be looking for (Granada! La Venta!). This is even better on a continents map as you'll likely be able to control the reveal of each continent (assuming the map generates 3+ continents, which it often does) so you may want to become allies with someone on one continent in one era then hold off and do it again for someone on the other continent the next era. Conclusion: best unique improvement in the game, allowing you to grow production very early on while also growing your cities faster and providing all the housing you'll ever need (Indonesia's Kampung, but more useful since it is built on land). This is especially useful now given how city size affects loyalty. The trade routes/territory ability is better than I expected allowing faster, easier and more controlled expansion while filling your coffers and the allied ability not only provides decent era point improvements, it also provides some mild but noticeable improvements to the new alliance system.
2. Cyrus - Persia
Cyrus is clearly extremely powerful because of his war-related ability affecting unit movement. However, I tend to be defensive (though I'm getting more feisty lately!) and am much more interested in a cultural victory than domination so this is less important to me than most. The Pairedaeza is another amazing unique improvement. Plaster Pairedaeza and you will find yourself generating enormous amounts of culture and gold. Once flight hits, they help push a cultural victory along nicely even in a smaller empire (say, one focused on great works and wonders). The immortal is also very useful for quite some time and is easy to make early on. And it gets better over time! They can generate enormous amounts of gold and culture early on. Persia can move through the culture tree faster than most anyone (except, say, Pericles) because of this improvement. If you can ally with Granada or La Venta (which both allow for improvements that produce culture if you are suzerain) you can use those improvements to fill in the spaces between your Pairedaeza at which point you'll rip through the culture tree and be virtually impossible to beat once flight rolls around. The immortal can help Persia focus on building up improvements and districts by providing quick, strong and early protection.
3. Lautaro - Mapuche
I am beginning to enjoy more offensive games as I feel the loyalty mechanic made it more enjoyable. Mapuche is perfect for this given their ability to produce units that gain experience faster, their strength bonuses when close to home and their ability to quickly whittle a city's loyalty down thus allowing one to take more powerfully guarded cities with relative ease. Mapuche are great at playing a game comprised of a long series of short expansionist wars where you quickly take down the closest bordering cities while sending units ahead to both raise havoc with loyalty in more 'internal' cities and plunder, plunder, plunder. End the war as soon as the option is available (assuming you have taken the border cities. Wait a short while, building units and districts while your neighbor(s) rebuild all the plunder damage you caused. They will be weakened and fallen behind due to the city losses and the improvement damage. Rince, repeat. Finally, the Chemamull is an interesting unique improvement. It can be quite powerful, though it can be hard to place (especially early on and/or if you tend to have lots of mines). However, a handful placed early on is enough to provide a decent culture boost (though not one as strong or reliable as Persia's). If you can get the Eiffel Tower, the Chemamul can become a powerful component of a cultural victory as you can place many more, and these will produce large amounts of culture).
4. Robert the Bruce - Scotland
Scottish Enlightenment is really great. Production, science, great people all come faster, stronger and earlier. The golf course is more fun than I thought and can provide a nice, quick boost to culture and gold if you've planned your cities properly. However, it is one of the weaker unique improvements because (a) it can only be built once per city so no spamming it, thus rendering it's overall output much lower than other unique improvements, and (b) it comes later than many unique improvements. The other aspects of Scotland don't interest me much (though love the bagpipes until the sound glitch begins!).
5. Catherine - France
I'm learning to like spies and France allows me to play with that aspect of the game more (an aspect of the game that I hope the next expansion fleshes out, btw). France also received a good boost in R&F. Not only is the increase in diplomatic visibility much more powerful on its own than it used to be, it is further strengthened by the added combat power France gains from having a higher diplomatic visibility than its opponent. France also encourages wonders and their ability to increase tourism from each wonder makes the somewhat overall weaker power of wonders in VI much more valuable. This makes racing for and building many wonders especially useful. The unique improvement is one of the weaker in the game, especially given that it comes later, however it does provide decent culture and can provide exceptional culture output if planned correctly. Unfortunately, it is a much poorer version of Persia's unique improvement.