Until recently, I would have said chariots. My most recent Bannor game changed that, what with the large continent I've got to travel across.
The unit I don't like building (but still do occasionally) is the catapult. I'll look for any other solution first, be they highly promoted super units, mages with the right spells, priest, or excessive fodder units. Why don't I like them? They are slow and never seem to get the option to go faster. Even once they get where they need to be, they have to wait a turn before they can be useful since it takes move to bombard. The collateral damage can be good, but it takes a good number of catapults to cause the same effect as one maelstrom/ring of fire/tsunami and the catapult is consumed in the casting often enough to require a steady stream of replacements. Even when they do survive, that's several turns of healing, slowing down any military advance further. If I could get them a few more promotions, I could mitigate that a bit. However, with their inability to kill anything, their xp gain seems really slow. Then there's the enemies assassins picking them off. Ugh...
Like I said, I still build them. Sometimes its the only option against early archers in opposing, fortified cities. I just really don't like using them.