Played them all and have to say Rome hands down. For one it didn't have the pope system which stopped you fighting other christians for goddam ages. And unlike medieval II I thought the stronger units actually showed they were superior, since medieval II if you had really strong heavy infantry against a unit of weak light infantry the casualties on both sides seemed a bit random. Also as someone mentioned there is excellent variation of units at the start in Rome and good variation of unit purposes. Also no castle/village system - that thing drove me up the wall since you could make an awesome castle but by the time you have it's too far from the front line to be effective anymore.