Sorry about this old bump, I'm experimenting with finding old threads as the board doesn't have a next page button - and also these threads are great and I wasn't around at the time to contribute.
I agree fully with spoonwood here, Cavalry are by far the best unit in the game. I can't speak for spoonwood about the cause of this but my reasoning is as follows:
1. They are the only non-Unique Military Unit to have three moves per turn at a point in the game when moves per turn makes a huge difference.
2. They can be attained only half-way into the Medieval stage and can not be properly defended against until half-way into the Industrial age, the most notorious AI tech slow-down phase.
3. Combined with Roads they can get to even the furthest distance on the map in just a few turns making reinforcements a simplicity rather than a calculation.
4. They can stay at a safe distance, attack, retreat to a safe distance all in one turn.
5. Once in an army it takes a monumental RNG fail for them to ever lose to anything other than Fortified Veteran Infantry.
6. They have awesome sound effects. Clip-clop, clip-clop, neeeeigh, click-boom, click-boom, click-boom, click-boom, enter city, yeee-haaaa. Lol.
7. They are so easy to use that even the AI can be devastating with them!
There are tons of other useful units in the game, all with their pros and cons, but I think most of them depend a lot on the setting and game choices. I'm hard pushed to think of a scenario where a stack of Cavalry isn't ideal whatever the situation. Even in the Modern Age a Cavalry Army can give a Mech Infantry a hard time.