As with many of the above suggestions, go for Rome.
The variation among the factions & units is broad, and the modding community is very good. I regularly get the updates for Rome: Total Reality, and it is one of the best games I've ever played. Medieval 2 is run off the same engine, but the expansions were more varied than for Rome (Barbarian Invasion excepted).
Empire & Napoleon have a vastly different feel, with a simplified building & diplomacy interface, and a few other different mechanics, but overall it's not much different except 'guns' and playable naval battles (the biggest draw of it, IMO).
Shogun 1 and Medieval 1 are still good games, except with shit graphics. Shogun 2 is pretty good (playing it atm) if a bit monotonous in mid-campaign. It is good in that the late-campaign is very frantic unlike all the others.
Or, you can wait for the new one to come out (granted, probably in a year and a half's time) and get your opinions off that.