In Civ IV I've always loathed Zara Yaqob. He always grabbed a lot of land and got ahead. Adding insult to injury, he was one of the "random" opponents in 60% of my games. I still hate that guy, too. In Civ V, I don't know yet, haven't played enough. Anyway, if leader personalities are as plain and unpredictable as some have said, then may be I'll never have a usual nemesis like I used to.