I'm only early in a Julii campaign, so I have not seen the better units yet.

I am inclined to say the wardogs, as they are rather funny to watch and incredibly powerful against Gauls plus they regenerate. But their power, regeneration and indeed existence seems ahistorical and takes some of the fun out of it.

Other than that, the one unit of Roman archers I started with has earned its weight in gold. They are amazing against the (admittedly unarmoured) Gauls - they seem even better than Samaurai archers in STW. To be honest, they seem too strong - it is as if they were good shots armed with rifles (not muskets, rifles). However, this is somewhat balanced by the ridiculous charge speeds and the appalling rate of friendly fire.

Of the AI units, the Dacian falxmen earned my respect - cleaved through a unit of hastati until my wardogs hit them - and the AI General's bodyguard cavalry are very much like the royal knights of MTW (one of the few dangerous Gaul units under the AI).