Actually, I suppose it comes down to the amount of actively producing villages. No village will produce iron faster than another.
Yeah, but to much villagers where i have it create other problems. i have limited mine around 20. it generate enough for the things i need it for. (far below enough for building with it)
I think that Tzgiles made a fantastic one on SMP 3. I do not know the rate of iron an hour (cause I don't just hang around there that long), but he has around 12 spawning pads set up. It is very good for my needs, and it's public.
BrenJone and I use a 21 cell farm. I haven't used it in a while so I don't know its rate, but I believe it can get about a DC every 2-3 hours