DRAGON:
These two have very clear choices. If you are running Archers or Siege you will want Courageous.
If you are using Ground you want the Fearless.
And lastly if you are using Cavs you would choose the Courageous for defense and Fearless for offense.
ARES:
Ares is going to follow the same guidelines: If you are running Archers or Siege you will want Courageous.
If you are using Ground you want the Fearless.
And lastly if you are using Cavs you would choose the Courageous for defense and Fearless for offense.
ACHAEMENIDAE:
Here you are looking at the Courageous for a range heavy defense. Also useful if you are trying to catch someone who is Cav heavy.
For Ground and Siege based activities you would want the Fearless legs.
With Cavs you're going to want to go with one of the Ares options unless you are going for the full Achaemenidae set on your Cav gen (which I don't recommend). If you don't have or can't make Ares legs and you are running Cavs you would want the Fearless to debuff Cav troops main killers: Range and Siege.