That's a very complex question.

You'd have to define "richest". Are you talking about disposable income, or some kind of total aggregate wealth?

You'd also have to explain "sovreignty". Does this refer to the ruler (or sovreign) alone, or include some or all of his subjects? Does it refer to a state as a whole, including the wealth of its inhabitants - or should there be some kind of assessment of how much capital would actually be realisable/mobilisable by the government?

Also, are you asking for the wealthiest state of all time, or do you want some kind of running measure - i.e. who is richest at any given point in time? What about wealth relative to size/population of a country?

If you are happy to go with an aggregate value for the total wealth of a country, as measured by Gross Domestic Product (GDP - the aggregated value of goods and services produced in a country in a given year), then Angus Maddison's dataset (link to 2009 horizontal set provided) might be your best bet. He has made considerable effort to make his figures comparable across time and space.

That said, it should be noted that there are great difficulties in calculating GDP, and that these difficulties are compounded by time and lack of reliable public records. Maddison's data for the pre-early modern period especially should therefore be taken with a solid pinch of salt.

edit: In case you are unfamiliar with such datasets: Open the file in excel and look at the spreadsheets. You have different sheets giving GDP, population, GDP per Capita (GDP divided by population) and more. The individual sheets contain horisontal tables listing countries (and special regions/areas) and years, allowing you to compare data over time. Early parts of the data have many gaps indicating that Maddison has found it impossible to calculate annual estimates sufficiently accurate for inclusion (in other words, we don't know). Values for GDP and GDP per Capita is given in 1990 International Geary-Khamis $ (a standardised way of caculating sustained value over time), allowing you to compare numbers from year to year without worrying about variation in the value of money.