The scientific studies are the ones based on survey methods, which were published in peer-reviewed scientific journals.
The Iraqiyun estimate was not published and didn't explain its methodology. The Iraq Body Count isn't a scientific survey - it keeps track of deaths that are reported in the media. It's highly unlikely that every (or even most) deaths that occur in a war in a country like Iraq (recall how dangerous Iraq was at the height of the war) will be recorded in the media.
That leaves the ILCS, the two Lancet studies, the PLOS study, and the Iraqi Health Ministry's estimate.
The ILCS was carried out only a year into the war, before the height of the violence (which was 2006). It was not focused on determining deaths - the question about deaths was only one of many questions asked. Nevertheless, this study found a far higher death toll than IBC.
The Iraqi Health Ministry was run by one of the parties to the civil war, so it's questionable whether people would have felt are liberty to speak freely to the Ministry's workers, especially about something as sensitive (and potentially incriminating) as deaths in the family.
The first Lancet study was also carried out relatively early in the war, but found a much higher death toll than IBC. The 2nd Lancet study was carried out in 2006, after violence had exploded, and found a number of several hundred thousand, but with large error bars. The later PLOS study was much larger and had smaller error bars, and found a number around half a million.
> the most covered war by journalists in history. The surreal feeling at the time was that it was all televised. It seems unlikely most if any were infiltrated by US covert intelligence white washers, but I can't fathom any overarching reason for such wide variance.
I don't know how Iraq compares to other wars in terms of coverage, but most of the country was not well covered. Western journalists were heavily concentrated in the fortified Green Zone in Baghdad, or embedded in US military units. During the height of the violence, it was extremely dangerous to wander around Iraq as a foreigner. Every study (even the one by the Iraqi Health Ministry) found death tolls far in excess of what IBC documented from media reports.