Alexander and Peloponessian War: Too simplistic and linear. It's all about building a big land army and conquering in a predetermined way.
Genghis Khan: Quite fun. I like the idea of teching by conquest. And it's a good game for trying out different Great General strategies. I just wish the camp units produced units with higher XPs.
Omens: An interesting idea. It's a little silly (like some of the weird mods with elves and orcs) but it's not boring like Alexander. And the French leader, Marquis Du Quesne (spelling?) has the ever covetted Aggressive/Charismatic combination.
Chinese Unification: Probably the most complete scenario - a great combination of military, research, building, and diplomacy. This has something for everyone.
Barbarians: A great scenario for players who love conquest, and each game is different. After getting my ass handed to me a couple times, I learned the value of pillaging and the power of trebuchets.
Best scenario so far? Chinese Unification.