What is the most overpowered unit in the game, and what is the most underrated? This in in relation to how they perform against other units and especially in auto-resolve.

For me, I think it is either the Bull Warriors or any kind of chariot units for overpowered. Chariots swing any autoresolve heavily, and Spain should not get a double hitpoint unit, especially with the stats that Bull Warriors have.

For underrated, I think regular Hoplites. They get no love in an autoresove situation, and they will hold their own against most other units.