Camel Archers rock, especially if you were unlucky enough to draw the "No Horses" card....
My vote is for Ironclads. Okay, they're tough, but they can't leave the coast. Furthermore, by the time I can build Ironclads, I'm almost to Destroyers.... Every other unit in Civ4 has its uses (Explorers make good field medics, for example), but Ironclads are a total waste of hammers.