Early days: Chariot rushes to clear out neighbours, noobs, or wonder builders.
Established start, classical: Axemen with 1-3 spears, possibly preatorians.
Construction era: Catapuuuuuults rule supreme. Put in a few elephants and axemen to lessen the blow from the othser sides cats. Its just one big catfight lol.
Trebuchet times: Trebuchets are nice when they are at the city, but they will never get to the city. The reason is 2 trebs lose totally against the 3 catapults that those hammers could be used for. The rest of the stacks being equal, trebs totaly suck totaly and suck totaly. The reson for this is that only 5-10% of the map is cities, and the trebs are dead fish in a barrel anywhere else.
Gunpowder era: Mustketmen are nice, but knights are nice too. these days dont last, and lead to:
Chemistry era: Grenadiers and cannon, very battle fieldy like lol. The cannons are of course the main offender here, while the grenadiers are useful to get defensive bonuses in towns and on the fields. These is great until:
Infantry era: Infantry with maybe a few machine guns, cannons, until artillery comes. The artillery rains supreme until marines and tanks.
etc.. by this time the game is usually over, and if its not, ur all a bunch of noobs! lol
In general, siege units are the best, simply because collateral damage kills everything.