Ramesses II has it all, great starting techs, great traits, good unique building and great unique unit, but it requires Horse which isn't that common on some map types.
So I went with Zara Yaqob, primarily for his Oromo Warriors with free Dill I and Drill II promotions. With Theocracy and Barracks, they can be drafted and given Drill III or they can be organically built or whipped and given Drill IV at only 5 XPs. These units are awesome as they are and just need some help versus units that ingore first strikes like Knights. Best of all they can be upgraded to Riflemen and retain all their Drill promotions.
Suleiman is a good choice too, but Janissaries that are upgraded to Riflemen lose their +25% bonus versus melee, archery and mounted units.
Qin Shi Haung and his Cho-Ko-Nus has a great trait Industrious and Protective that many players do not appreciate as much as they could. In this case, protective adds both Drill I and City Garrison I to archery units. So Cho-Ko-Nus get a free offensive promotion, Drill I, and a free defensive promotion, City Garrison I. Of course colateral damage is what makes Cho-Ko-Nus so great. The can destroy enemy stacks in the open versus their strength of 6 over Catapult's 5. They also get the free Drill I promotion which makes the Drill line much easier to complete (10 XPs versus 17 XPs). The Combat line might be better at causing enemy damage, but Drill greatly reduces damage one's own troops sustain. Also, Iron is usually plentiful, especially when Iron Working is prioritized early on.
Sun Tzu Wu