The concensus seems to say either the Celtics or the Iroquois are the two best Aggie civs. Both are also similar in that they have two of the best early units in the game - Gallic Swordsmen & Mounted Warriors. So, it depends on your preference of their second trait: Religious for the Celts vs. Commercial for the Iroquois. I've actually never played as the Celts (the Random Setting Gods haven't been so kind as to bless me). But to me, religious is just middle of the road - useful & better than some traits but not as important as some other traits. In my opinion, the Iroquois is a 1st Tier civ - the Commercial trait is severely underrated by many as its benefits aren't immediately obvious (i.e., it's not the "immediate gratification trait" such as giving free techs at the start of the era or half price buildings). The benefits of being Commercial start to manifest when your empire get bigger, especially after getting out of despotism. But it's more of a late game power - when many of your cities will be less corrupted in shield & gold production. And it's that slight savings of 1-2 gold and shields (excluding the additional gold bonus in the center of every city) that allow Commerical civs to churn out many things - buildings, units, wonders - slightly faster, while funneling more gold into research/spend on other things. You can color me biased since I'm playing the Iroquois right now. But then again, I've never played them before (or considered playing them before) this current game. They were assigned as my random Civ and overall, I'm impressed - they can both be an early game power with the Aggie trait and the Mounted Warrior unit, as well as a late game power (I'm in the late industrial age in my current game).