1) Alpha Centauri (Classic. Spent more time with this than any other game)
2) Medieval Total War (Got me interested in history, and was a great game to boot)
3) Fire Emblem: Path of Radiance (only one of the series I've played, others may be even better)
4) Civilization 4 (Loved what I've played thus far, but I've had far too little time lately to start properly)
5) Europa Barbarorum (getting close to what RTW could have been)