Perhaps I might start a "conversation" with you to further discuss some other things like unit Attack and Defence values, or you think it would be more useful to do so here?
Thrust123... If you want to discuss Attack and Defense values, probably a "conversation" would be better. If you have specific questions that you believe others would be interested in, here is fine.
Generally, Attack and Defense values are personal to anyone setting them for their game.
Same for Movement settings and game play factors that depend on what is wanted.
If you are speaking about Attack and Defense values regarding what has been called "Balanced Game Play", remember to have a fluctuation in balance because if everything were always in "Balance", nothing could be gained. This would be like a stalemate in Chess. A Fluctuation in balance can be created in many ways. It can be the difference in the numbers of units, fast or slow movement, unit costs, abilities, Unit Support cost or none, Upgrade abilities as well as their Attack and Defense values. Also, Improvements and Wonders that provide many benefits can be very powerful.
Ask yourself "Do I want Hordes of units with less Attack and Defense or fewer units with Very Strong Attack and Defense?" Both have a place.
What ever Attack and Defense values you choose for units at any time for game play, make sure to build in creative methods for being able to Attack and Defend. A variety of methods will allow players the ability to play according to how they want to play as well as game replay ability.
The Strongest settings are Defense and Hit Points because no matter what the Attack values are, if the unit does not have Good Defense and Hit Points, it can be destroyed easier than units with high Defense and Hit Points. This is especially true given the "Random Play" factors programed into the Civ Game.
So what are the Best Attack and Defense Settings? That is a very loaded question that is totally dependent upon the game and what is wanted.