I think the +1 food from water tiles is the best civ trait. The aztec combat bonus doesn't matter much when I've got superior units due to superior science and growth from all of my coastal cities.
With Aztec's defending units coming from barracks, garnering promotions, and entrenched in defensive positions, you're at best matching the defenders' stats with your green units. (Only exception is if you're lucky enough to have a Great Leader.) While the defenders would have garnered a lot of experience from the attackers suiciding for naught. On the other hand, when Aztec shock troops go on a capturing spree, their tech rate should also rise. Even worse for the defenders if they have blitz. They keep marching from city to city without stopping to rest, giving the defenders no time to recover.
Rush a temple in each new city and watch their culture, science, and production rate rise simultaneously. Temple giving +3 science is a very underrated, underestimated trait. In OCC using the Aztecs, my lone temple was the catalyst that allowed me to outrace the other civs--including Chinese and Greeks--in techs. So not only do they eventually get the same or better units that you claim to have, their units have much higher survivablity and much more destructive force.
Food from water tiles is a nice trait, but the traits in my list are actually game-breaking. Better than 1/2 price wonders? Better than +50% gold production? I don't think the terrain-dependent traits as they stand can qualify as the best traits, unless they double their effects. If it was +2 food, that might put them in contention for best trait.
The major downside is terrain traits don't affect all your cities equally. You're not going to have a lot of the tiles that match the trait. Sea tiles probably average less than 50% of your cities across the entire empire. It's even worse if you're looking to capitalize on plains or mountains.
Secondly, growth rate can be increased in many ways by everybody. Granary, aqueduct, Hanging Gardens, harbor. In other words, the terrain traits aren't that much of an advantage and aren't quite unique in the sense that other civs can acquire the similar effect. Food in sea tiles? Harbor. Extra food in plains? Granary. etc. Something like Zulus' medieval trait that makes cities grow faster is more consistent and relies less on lucky tile setup.
In short, terrain traits give you that extra 10% or 20% efficiency. Whereas certain traits like 1/2 cost wonders and auto-healing units completely turn the game in your favor.
