If you are looking for the absolute "game-breaker," is widely considered the best tactic ever created for the CM 01/02 engine. It is a 3-3-3-1 / 3-5-2 hybrid that utilizes heavy "With Ball / Without Ball" (WIB-WOB) editing to overwhelm the AI.
: Set fullbacks to cross the ball and give your central midfielders forward runs (upward arrows).
: While originally designed as attacking, many veterans find it performs even better on Normal mentality to prevent fullbacks from being exposed by traditional 4-4-2 AI setups.