SlipperyJim said: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.
There's a couple of other factors that limit ironclad's usefulness, which I'm surprised noone's mentioned yet.
1. Unless you're behind researching steam power, you get ironclads at the same time you get to build drydocks.
2. If you're doing city specialization then probably most of your coastal cities are commerce centres or (if they have fish/etc.) GP centres. You may have only 2 or 3 coastal cities with enough production to make it worth even trying to build military units in them. This means you *really* need those drydocks if you're going to fight a sea-based war. There's a similar argument for the ironworks which also requires the same tech, though depending on your situation you might or might not want the ironworks in a coastal city.
Now combine those issues with the short tech interval to destroyers, and which would you choose when you discover steel? To start building drydocks immediately, knowing that by the time they are complete you'll be almost ready to start turning out destroyers, or to build ironclads now, knowing you're just delaying the time when you'll be able to really start building a strong fleet? I suspect most people will go for the drydocks, unless they are already at war and need naval defences super-urgently.