As indicated above, you will receive many recommendations for many different rule sets.

My own personal favorite is
Swordpoint, written by Martin Gibbins and published by Gripping Beast:
http://www.grippingbeast.co.uk/SWORDPOINT.html To help narrow the field a bit, you may want to ask yourself a few of the following questions:
How many figures do you plan to have in your "large" battles? Different Ancient-Medieval rulebooks for mass battles can call for armies with as few as 24-48 figures per side on the low end, to forces having as many as 200-450 figures or more. Swordpoint recommends battles with roughly 100-250 figures per side, although larger armies will also work perfectly well.
What kind of turn system do you prefer, IGO-UGO, simultaneous, card driven, or something else? In Swordpoint, players dice for movement initiative, whereas shooting and combat are simultaneous.
Do you want rules that allow your tabletop troops to follow your orders automatically at all times, or do you like to have somewhat less control through the use of limited command points, throwing dice to see if your orders are followed, or some other form of command 'friction?' Swordpoint does not limit or interfere with your ability to command your troops, although you can attempt to influence initiative and combat through the use of Momentum tokens.
Do you enjoy rules in which combat and other tactical processes are dealt with in precise detail (process oriented), or ones in which many of those details are abstracted into a few factors and/or die rolls (result oriented), or something in between? For this Swordpoint falls in the midrange, having special rules to give various units appropriate abilities, with other details abstracted, e.g. representing the effect of Roman line exchange by giving legionary units more staying power in combat.
Are there any other rules mechanics that are definite 'must haves' or 'no gos' for you and/or your fellow players? For example, do you want a rule set that has many game mechanics that are similar to those in SAGA?
Hopefully you will find these questions to be of some help as you mull over the many available options. Best of luck in your search!
Cheers,
Scott