The rules in the GURPS Basic Set are really excellent (elegant and flexible, realistic, extensible, and capable of either very simple fudgey play, or very detail-oriented rules-based play). The hand-to-hand and ancient/medieval combat rules, played on a hex-map with all the advanced rules, are the most accurate and realistic combat system of their type that I've seen, second only to the mods I've added to them.
PvK