I tend to use flat drawn cable or long box chain, because I like the shape of the links. But I would be happy w/ just about any generic style if they are offered here!
I prefer lobster clasps to ring clasps. 1.5mm chains seem to be reasonably sturdy.
For silver pendants, I use sterling silver chain.
For bronze, brass, or gold plated pendants, I use gold filled chain, rather than gold plated.
For steel, I use steel ball or steel cable chain, which is available in a variety of colors.
18" is the most common length requested but I have a couple of pendants that I created for long chains (24" or 30") so for those, I don't offer a choice. But if just 18" were offered here, I'd be fine w/ that.
A visit to a couple of the wholesale shops in the jewelry district is probably worthwhile to check out the variations in chain design, thickness, and clasp choices.
@Ontogenie - your pictures and pendants are beautiful!