I can only talk about regular Civ IV, no expansions.
Monty. He's predictable but that still doesn't mean that he's weak. And 6 longbowmen on a hill city with good coltural defense and walls can kill off 30 units, so he brings 100


![Pissed [pissed] [pissed]](/images/smilies/pissed.gif)
. And on Deity, forget about fairness, so you get

from him.
Mansa. He expands REALLY fast and also founds religion early. He's tough to bump off early because he has Skirmishers, and later in the game, he generally is rich enough to bribe friends into a war. He's good far away though, because he trades techs more readily that others.
Isabella. She is a B**** when it comes to religion. If you have a different religion from her for 1 turn, she's already at -4 for religion. And she's kind of a wonder freak.
Gandhi/Ash. This is an interesting situation... the two most useful wonders in the game for me are the Great Library and The Pyramids, and both Indian civs tend to streamline towards them. If I have them, then I have no trouble at all with them.
Saladin. He's a wonder-builder, and he annoys me because it sucks to be 1 turn away from finishing 3 different wonder and then have him finish all 3.
Toku. VERY often when I play as Hatschie, I end up right next to him on Deity lvl. He expands fast, but he's an isolationist, so basically, I can't go anywhere past his lands, since the jerk doesn't give me Open Borders until I'm at about +8 with him.
I don't know why people hate Cathy. She gets to be really powerful, but she doesn't declare war easy, and if you're friends with her, she is easy to get a tech out of, and when she has cossacks, she DOMINATES in war (except vs. redcoats).