Are you looking for unit combat rules (involving miniatures and maneuvering and detailed resolution, or a quick-resolution system? If the former, I'd say
Cry Havoc (published by Malhavoc Press) is the best; it uses a ruleset that's basically just d20 writ large, so it shouldn't be too hard to learn. However, it does require running battles on a pretty detailed scale. For larger battles (and a nice set of rules on running a realm to boot!), I'd say a better choice is
Fields of Blood, by Eden Studios, which also features a pretty easy quick-resolution system, and has real rules on actually building, training, and equipping armies, which are nice if you want to create balanced forces for a conflict. That said, if you want something that allows you to run things REALLY simply, I would recommend the War Machine rules published way back when for the D&D Companion Set:
http://www.rpgnow.com/product_info.php?products_id=1198&
It costs $5, and the rules are easy to apply to 3e. Sure, they're not very precise modeling of a D&D combat, but they work well enough and they are VERY simple to play.
