I have four of the mentioned settings (Iron Heroes, AE, Ravenloft, and The Black Company).
I've played/DM'd none of these four yet (I will someday when I have more time....seriously...I will...), so my judgement is based purely on what I've read.
I'd currently rank Iron Heroes as the best. The appeal to me is that I can finally run a low magic campaign without taking the "power" out of the campaign via the expanded feat system and class abilities. I also find the diversity in the classes very appealing. This will more than likely be the next campaign I run...as so as I get time
A close second, would be The Black Company for two reasons. One, my group and I loved the books. Two, the magic system is pretty cool.
Third, I go with Scarred Lands (which I don't believe was listed) only because I have about 20 misc Scarred Lands books. Well maybe not.....third would probably Ghostwalk, because it would allow me to resurrect an old campaign I was DMing that has be on hiatus for 2 years by killing the PCs and having them return as ghosts.