Hard question.
It would have to be a civ that I'd want to include to start with, and, then, it would have to be a leader that I'd prefer to have over any other possible leader for that civ.
That really slims down the list. For Brazil, Lula might be a good leader, but over Pedro II or Vargas? Mandela would have awesome potential, but I'm not sure about South Africa as a civ.
So, post war leaders of civs...
Venice makes for a great civ, but what about the Holy See as a different kind of small civ? John Paul II might not be the very best of all popes ever, but certainly made a huge mark on the world and is quite recognizable. Enough so I could give him the nod. UA would simply have to be something about religion. Maybe cause fairly severe unhappiness when at war with the Holy See (as he was a fairly political guy).
Charles de Gaulle could make his return to France. Not that Napoleon is bad, but, come on, there are plenty of leaders of France, and de Gaulle is as good as any to be a part of a cycle. His UA could have to do with friendships or alliances, and probably cultural, as France tends to be. Hard to say since Civ games tend to change a lot with each new sequel.
Finally, and this is a stretch, introduce Chile with Pinochet. I'm not crazy about the civ, but, really, the well runs dry pretty quickly. I feel like it's a lot like introducing Hitler to Germany though (both in I don't feel Germany should probably be a civ and that Hitler, while perhaps the single most important and effective ruler, is a bad choice for obvious reasons, I feel the same about Chile and Pintochet). For a UA, let him flat out ignore the effects of minor unhappiness. Not that it counts as happy, but no loss of growth, production, military strength, golden age points, etc.
All of those UA's are based on the person, as well, though many UA's are based on the civ. Elizabeth has Sun Never Sets, but is before the discovery of the Americas. Sweden is known for the Nobel Prize, not any leader, and certainly not one from before Nobel was born! Isabella similarly has a UA that refers to a moment well after when she lived, and Germany, a time before the nation, or the kingdoms that would sit on that piece of land, even existed! But I like a UA that goes with the leader, not just the civ.