Game changing:
1. Assyria - the new definition of easiness;
2. Portugal - very unique gameplay;
3. Zulu - extra promotion from barracks;
4. Poland - extra social policy per era opens new paths;
5. Brazil - generic civ;
Challenging to play against:
1. Zulu - they have nothing, which AI can't handle. In addition, they will be agressive expanders (with uber bonuses). Second Iroquis I guess.
2. Assyria - strong science and I quess they will be programmed to early rushes with UU.
3. Poland - only because they will probably be aggresive; (though it would be great if this civ would have truely random attitude)
4. Brazil - really small chance to be a problem; even if they start on the second corner of map you can still nuke them; late game bonuses don't help;
5. Portugal - AI will not be able to handle it; they will spent extra money on City-states making UI useless; Money and Happiness is nothing which any AI civ truely lacks;